When typing it overwrites existing text. Will not Insert

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 - should also be valid for later versions…).

Hit INSERT key again and this will cure your problem… - Cheers