#1) Remove screws from needle plate, 2) It may help to remove feed dogs, for me I leave them in, but there are two screws on the hook assembly shaft. Remove the two screws. 3)Pull hook out 4)Clean bobbin case assembly you may need to disassemble it if there are threads stuck that won't come out. You can remove the screws on the assembly if you need to do this. Put the hook back on carefully-hopefully you won't move the shaft and look for previous screw marks to line up the holes. You may have to adjust this to get it back in time correctly. I've done this many times. Good luck.
You will need to buy a bobbin case. Expect to pay around $50 for that.