According to the WWDC app, "Platforms State of the Union" starts at 2:30 PM PT in the Bill Graham Civic Auditorium.

Enterprise iOS Distribution Certificates are only good for 3 years. You will need to create a new certificate, assign it to your distribution profile, and rebuild the app using the new cert and profile. Log in to your developer account, go to "Certificates, IDs, & Profiles". Click + on the Certificates page to start creating a new certificate. Once that is ready, go to the Profiles page, click on the app's profile, click on Edit, select the new certificate, and click Save.

Don't revoke the old certificate, or you apps will immediately stop working. You can have 2 distribution certificates. Create a new distribution certificate, update the distribution profile to use that new certificate, then rebuild the apps using the new profile and certificate. When the old certificate expires, I think it will be removed automatically. When your new certficate expires in 3 years, repeat the process.Edit: Is your certificate expiring, or the distribution profile? Certificates are good for 3 years, profiles for one year. If it is the profile that is expiring, you can edit it and click Save (without changing anything) and it will be renewed for another year. Rebuild the app with the updated profile.

First thing to try would be delete it and download it again. There might have been a glitch when you downloaded it the first time.

On the Certificates, Identifiers & Profiles page, there should be a blue + symbol at the top of the list of certificates (after the list header label "Certificates"). Click the + to start the process of requesting a new iOS Distribution certificate. You will want to choose the "In House & Ad Hoc" option. After the new certificate is created, you will need to edit your distribution profile (click Profiles on the left of the Certificates, Identifiers & Profiles page, click on the profile, click Edit), select the new certificate, and save it. Then rebuild (create archive and use Distribute App button in Organizer window) your app(s) using the new certificate and updated distribution profile.

You need to click on a specific profile, then click Edit. The available certificates should be shown at the bottom with radio buttons to choose the one you want. If it doesn't show up, are you sure you created an In House and Ad Hoc certificate and not a Development certificate? On the main Certificates page, the TYPE column should show "iOS Distribution".

Do you have an Enterprise account? You used the Enterprise forum tag. If so, the new certificate should be valid for 3 years (2023/07/02), not one year. Or they used to be anyway. I only have one distribution cert in our Enterprise account right now, and I don't want to use my second one yet, so I can't test creating a new one to see if it shows up as a choice in my distribution profiles. You shouldn't need to create a new profile. In the past, when I have had a second active distribution cert, both have been visible in my distribution profiles. Hopefully someone else can help.

When you install an Xcode beta, it is named "Xcode-beta" and your previous version (Xcode 11) remains installed as "Xcode". When a new version of the beta is released, delete the old beta and install the new one. When Xcode 12 GM (Gold Master) is released or the final version is put in the App Store, delete the beta version. If you want to keep Xcode 11, you can rename the Xcode application to something like "Xcode-11" before installing the released version of Xcode 12, which will install as "Xcode". To rename, just go to the Applications folder, find Xcode, and rename the file. If you don't want to keep Xcode 11, I think you just install the release version of 12 and it replaces 11.

Apple has really been pushing Custom Apps distributed through Apple Business Manager (to your own company) instead of Enterprise Developer apps. However, as I understand it, Custom Apps / ABM doesn't work well if your employees are in different countries, as "redemption codes can not be used across countries or ABM instances at this time" (see this thread - https://developer.apple.com/forums/thread/130589 for example). So in that case, it seems like Enterprise is still the way to go.

To go back to iOS 13, you would need to restore your device to iOS 13, then restore from the backup you created while it had iOS 13 installed. Google something like "revert from iOS 14 beta to iOS 13" for instructions.

The plus symbol is in a blue circle after the word Certificates at the top of the list on the main Certificates page now. If it isn't there, you might not be assigned to the Admin role.

No need to regenerate/redistribute app. As long as your account is renewed, the apps will work until May 2021.

OTA still works, that might be what they are referring to with "using the iOS App (IPA) file".

You can have two active distribution certificates. They are good for 3 years, so you normally "stagger" them by creating the second one when there is about one year left on the first one. You can revoke one before it expires, but then none of the apps signed with it, or apps using a distribution profile signed with it will run.

Also, you can have as many distribution profiles and apps as you want. They can use either of your active certificates.