Problem in monodevelop (not technical )

Hi,
I have started learning scripting a few days ago and Iam using mono-develop ,so ...When I type something and I want to change it I go back in the middile of the word and when I try to change it ,it delets another letter ( Replace ) because my cursor is like a block !! .
Example : If wrote "fooball" and I discovered that its wrong so when I try to get between letters the block cursor selects a letter (it must select a letter ....Ex if [] is the block cursor then ..."fooall or fo[o]ball ) and then I will have to write the word again.
If you dont understand what I mean then see this image and just tell me how to change the cursor from the red one(the one that I dont want ) to the blue one (the one that I dont want ).


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Press the Insert key on your keyboard to switch back to normal typing mode.

 

Thanks very very much !!!

 

It didn't work with me until I pressed function + insert buttons.