microphone is not working how to fix it

Sadly its a bear to get it out to replace as you can see reviewing Step 13 here: Apple Watch Teardown

The next option is try cleaning it from the outside. Using a cotton swab (Q-Tip) wet it with 90% or better Isopropyl alcohol pressing the wet swab into the hole a few times to drip the alcohol into the hole. We don't want to insert anything into the hole as you could easily damage the microphone. Then using a can of can'ed air holding the nozzle a good 1/2 inch away from the hole directly inline one or two very short blasts to create a pressure wave to push the alcohol in then out. You may need to repeat a few times, testing the mic after each application.

Sadly this still might not work.