When doing a puzzle, let’s say 2 squares =5, and you know it has to be a 1 and 4. We don’t know the order, but put in 4 and to help remember. Later we realize they should be switched. Tapping any cell and clicking a button should Replace the contents of that cell, not append. In this example, I tap on the 4, and want to hit the 1 button. But instead of replacing the 4 with a 1, we get “41”. Now you have to hit delete, then type 1 again.
It would be much easier for the users to simply have the behavior be tapping a cell, then a button always updates the cell. That would help also if I accidentally tap 3. I could just tap 4 and it would update. NOT give me 34, or make me delete the 3.

Hope that makes sense. I think it would make solving much more fun, and much more realistic.