Generally when cursor is placed between words for example and you start typing it will INSERt the new words.
All of a sudden it will only overwrite.
What setting it this and how do I change it back
You probably hit the INSERT key - then you got the overwrite mode. Check the status bar: If overwrite mode has been chosen it will show overwrite (or similar); if regular insert mode is chosen there will be no information (LO 6.2.5.2 - should also be valid for later versions…).
Hit INSERT key again and this will cure your problem… - Cheers