Need some help with "noob" problems.

I'm fairly new to PyCharm and I've met this bug that makes my writing very weird. Basically, when I write lines and I decide to step back and rewrite something it automatically replaces the old characters with the new ones. Here's an example, 

print("hello friend")

( i go back and i am at the | with my cursor and click )

print("hello |friend")

( then i would want to make it say "my friend" instead of "friend"

print("hello my end")

I really had a hard time trying to explain this but hopefully you understood.