There's a key on the keyboard, called insert. Press that once to change the setting from overwrite to insert. Press it again to change it back.
You can this call a 'toggle' if you like.
To help you see the 'state' Word is in, it's displayed on the status line (bottom of the Word window). See if you can find it!
https://internal.shenton.wa.edu.au/ITResources/Word/PDFs/Deleting,%20Insert-Overwrite%20Modes.pdf tells more.
When I'm in Word and I use the space bar it deletes the following character and I haven't figured out how to fix this. I have the same problem when I attempt to insert a letter into a word (for instance, if I type sae and attempt to insert a v to make save, it deletes the e). Does anyone know how to solve this?