There is an Additional Tagging and Reporting Tools plugin for MusicBee that can bulk-change text within a tag using RegEx formulas. I haven't been able to master it as it's complicated.
I always recommend the external application
mp3tag for bulk tagging chores like this. I do things like this all the time with it. After selecting the applicable files, you would...
1. Hit Alt+6 to bring up the actions screen or go to Actions->Actions in the menu
2. Click the "New" icon at the right
3. Name your action
4. Click the "New" icon on the next screen
5. On "Select Action Type" pick "Replace"
6. In "Field" select "Title"
7. In "Original" type "ft." or whatever it is you want to replace
8. In "Replace with" type "feat." or whatever you want to replace it with
9. Check boxes under as needed
10.Click "OK"
If you have more than one variation of "ft." that you want to get rid of, you would just edit the Action you created with the next variation as the "Original" in step 7.
As always, test your methods before doing 50,000 in bulk