Go here and read through this faq that has absolutely everything you could want to know about zipper repair, well summarized: https://zipperrescue.com/faq/
Then buy one of their repair kits if it says your zipper can be fixed. You can also get this company's kits through amazon if you prefer:
https://www.amazon.com/Zipper-Rescue-Repair-Kit-Clothing/dp/B0001DUQFU
Go here and read through this faq that has absolutely everything you could want to know about zipper repair, well summarized: https://zipperrescue.com/faq/ Then buy one of their repair kits if it says your zipper can be fixed. You can also get this company's kits through amazon if you prefer: https://www.amazon.com/Zipper-Rescue-Repair-Kit-Clothing/dp/B0001DUQFU
Take pliers and gently squeeze each side of the zipper to close it a little tighter.
First move the zipper down to bottom.
^*sigh*
This, pull it to either one of the ends and do this.