Flier vs Flyer

I'd come to a consensus on what exactly you're going back and updating.

If you're updating custom form fields, I'd just go to Custom Forms > Fields and do a search for any field with either of those spellings and update the field directly.

For flier/flyer in project names, decide how far back you need to update, if any. You can pull a task report or project report for either term with a timeframe filter and update any needed. Basically I'd decide what objects need to be updated and pull various reports/searches to help you find them. If you have Fusion I'm sure there's a way to change every instance across all objects from one to the other.