open terminal
sudo -i
enter root password
copy and paste this to command line:
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil
Unenroll by copying and pasting this to command line:
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil unenroll
Enroll in the DeveloperSeed Beta by copying and pasting this:
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil enroll DeveloperSeed
Close and re-open System Preferences > Software Update
Give it a bit of time.
The new Beta should now show up.
Thanks to @Steffen_Voglmann ^^