With small pliers, grab the flat part of the zipper main part, not the floppy finger tag. Don't apply to much enough pressure yo get a grip and just wiggle that main part clockwise, counter clock wise (as your looking at it) a few times. After a couple of wiggles, apply a little side pressure to un-zip as you continue to wiggle. Not much pressure with the pliers, not to much pressure in the un-zip
I found this will move the dust or salt and get it working again. Rinse it all with lots of water and a tooth brush to get the dust and salt out of the mechanism and zipper teeth.
I used to rub an old candle on the zippers, That is up to you.
I decided against anything long ago. IMO, Wax and oils just hold more dust and salt. Frequent use and fresh water rinses is my game plan.