This question already has an answer here:
I usually use scissors to tighten up the zipper.
You need to make sure it's tight enough to bring the sliders together.
When it is too damaged your only option is to replace the whole zipper part.
The zipper goes bad when
- the handle bends/abrading, or
- the zip is abrading
Most of the cases the handle can be squeezed a bit with pinchers and put some wax on the zip also can help.
But these are temporary solutions, the whole zipper needed to replace by a seamstress as soon as possible.