You can use iTMS Transporter.
There is a video from WWDC 2013 (session 306). I think there is also a video from 2012.
https://help.apple.com/itc/transporteruserguide/en.lproj/static.html
https://help.apple.com/asc/appsspec/en.lproj/static.html (note this has not yet been updated to remove the "hosted content" feature, which is going away at the end of this month).
Note that a limit of 1000 IAPs applies in some situations; it is simpler to keep below that. If you must have more than 1000 you will need to do some operations in batches.
Beware, this is not the easiest thing to use!
Also beware, I think the number of people using it is probably quite small and I consider it "at risk" i.e. Apple might turn it off at any time. You're also likely to encounter bugs, e.g. the App Store Connect web interface probably isn't regularly tested with apps with >1000 IAPs.
Good luck!