How do i edit ingame character?

I want to create something like a character editor .For example: i've got a pixel character ,and i want change his texture by editing ,for the next level,or draw my own weapon,and do it all in game (image) editor . Also how to make mouse move by grid? Thanks!

I think the first thing you want to try is highly complex and system demanding. But I'm not sure, maybe there is an easy solution.

Regarding the second thing... do you mean like the cursors move on consoles?

I think there should be easy solution.About cursor: game like "2dminecraft" ,cursor move like pixel to pixel,right? This is how i want