how to change from overwrite to insert mode in a text editor

[TungFree]

I type 2 fingered, and on occasion I unwitingly hit wrong keys and get into overwrite mode. That is to say, I try to inset text but instead it overwrites the text that follows it rather than insert it.
what causes that and how do I get out of that mode.

 

[MrChad]

Usually it's the INSERT key on the keyboard.

 

[TungFree]

ooooo
yep yep yep that is exactly what was needed )
Mahalo Plenty ( thanks very much -hawaiian style