Premier Pro won't export - gets stuck at 50%

never update in the middle of a project.  If you need to, create a bootable clone of your startup drive using something like Carbon Copy Cloner

https://bombich.com/

This way, you can quickly restore your system and get back to work.  I know that doesn't help at the moment.  Hopefully you didn't check the "remove old versions" when you did the update, so you should be able to go back to using cc2019.

Now as far as the export problem.  There are a couple of ways to approach this.  there may be a piece of dicey media and you can export sections to find the problem clip.  Try exporting the first half of the sequence (using export sequence in to out).  If that works, export the second half.  If that works, bring both back into premiere and combine the 2 and try exporting that...  If that doesn't work, post back

Try exporting via Adobe Media Encoder.