Notepad++ replacing letters when I put anything infront

Sounds like you've pressed the insert key on your keyboard accidentally and switched to overwrite mode. Check the bottom right corner of the screen - does it say INS (insert mode) or OVR (overwrite mode)? Try pressing the insert key again to see if it fixes the problem. Alternatively, you can toggle overwrite/insert mode by clicking on the OVR or INS in the lower right corner.

You might have replace mode on. Try pushing the insert key on your keyboard to switch modes.