Well you could use the -i flag on the command line to modify a file, well at least in a one-liner.

Otherwise, I would not recommend opening files in read-and-write mode. But if you do want to do it this way, then you need to read the full file first and save the content in an array, make whatever changes you need to the array, and then only write back the array to the file. It works, but that would not be my recommended course of actions.

I would suggest reading the input file (read mode), writing to a copy of it (write mode), and then, if everything went OK, to do the house cleaning, i.e. deleting the old file (or archiving it under another name), and renaming the new one to what you need.