I have made many tag changes manually including album (I prefer the song point to the original album not some compilation), spelling etc. Every so often (sometimes after an update and sometimes not) the tag info reverts back to what it was originally on some tracks. How do I prevent this from happening?

I am assuming these are MP3 files.
Fix the tags again and right click the fixed song(s) and choose Convert ID3 Tags.
Choose version 2.3, and click OK.

This will write the tags that you updated to the files. Some updates don't get written automatically, and iTunes appears inconsistent on when it does and does not update the files.

Yes - mp3s. I had done this with many only to find some reverting back to what they had been previously at random times or following an update. This revert back can include comments that I had removed showing up again, the track # (total tracks) disappearing and the cover art gone.

I find that odd, as the Comment, Track # and image are all items that can be stored in the MP3 file directly.

I can think of two causes, the first is the MP3 file is read only and the tags are not being updated. Check that they are not set to read only on your hard drive. Then update everything that is wrong with it, including the album art. Then do the Convert ID3 Tags again.

Play the song and see if the tags remain the same or not. If they change, then they didn't get written to the file. You'll need to see why you can't write to the file. After read-only I would look at ownership of the file, or something else holding access to the file while you are updating it.

The second is that you (or someone else) are using Windows Media Player to play these songs and the options to update the songs with information from the Internet is turned on. WMP is updating the tags with the information it finds online. You can disable that "feature" in WMP and it will stop happening. Once you fix it again they should stay fixed.