Disconnect the battery first, removing the MAIN fuse should be enough but since some PO has been weirding up the wiring I would disconnect the battery "-" cable before doing anything with the red wire.
Those connectors have crimp-on spade females that lock into the plastic shell with tiny barbs in the terminal metal. You can remove the terminal part using a tiny screwdriver or something slid in beside the terminal to push the barb in so the terminal slides out the front. You can find replacements but getting one with the right barb for that shell is a challenge. If the old one is in good shape you can solder a wire on but it's best to carefully open the folded up crimped part to get the old wire out, fold it around the new wire and squeeze it as tight as you can with pliers without wrecking it, then soldering the wire in. No special solder but try to get old lead-zinc 60/40 solder: definitely not 50/50 plumbing solder, lead-free is OK if you can't get the 60/40. Use electronic solder with a flux core not "acid core".
If you can get an old harness at a junkyard you can harvest decent terminals (used, but not ruined) from the connectors on it. I have done this when I find a connector with burned terminals that can not be re-used.
