I was having some difficulty upgrading to 3.5.2, partly due to my hacking of c5 directories so I can install new sites more easily; however, to give back here is what I learned (this isn't a question/discussion, this is for reference for others - be sure to backup filesystem and database):

1) Upgrade link did not work, either /tools/required/upgrade nor /tools/required/upgrade.php

Nothing would happen in either case, and it would just return a screen "Upgrade Concrete5".

2) I decided to backup my existing database and c5 file system and decided to get all the tables fresh from a new reinstall. And tried step 1 again with the same result.

2) After looking at the upgrade.php it suggested to try /tools/required/upgrade.php?force=1

This resulted in a "Table 'btFile' doesn't exist" message and wouldn't proceed.


3) From here I looked at the line in the php where this error occured and noticed concrete5/concrete/themes/default is installed into the database explicitly looking for the inner_optics images. My edit of the default theme cause this problem. I reverted the default theme back to its original distribution files and tried step 1 again (having had to drop/create the database again)

4) Now I was receiving a getFile() object error, which I resolved by rm -rf /files/cache/*

5) Finally it worked and I restored my backup database; however, my theme was now missing and there was no content after the upgrade. I restored the themes folder and then via the Dashboard -> Site Wide Settings -> Debug -> Clear Cache and everything is now upgraded and fully populated and displaying correctly.

I hope this helps others!