why do the words erase when I write?

[Blue Angel]

When I go back to correct the words that I mispelled instead of adding
letters it erases them. It writes the letter I pressed but erases the one
next to it. How can fix this?

 

[Genine]

You need to switch on insert mode. Press the INS button on your keyboard, or
click the OVR on the status bar (bottom of the screen) as this toggles
overtype on and off.
Genine

 

[kelly]

press the insert button on your keyboard to change that
problem