if i want to re-play the game and make different choices, how do i do so when monika's character file has been deleted and she doesnt want to come back?

You have to re-download the game, meaning delete from your computer and coming back and downloading it again.

I hope this was helpful to  you.

Or you can just delete the "firstrun" file in the game folder

Yeah, either 'Re-download' the game or delete the file 'firstrun' found in the folders.