Mountain bikes, if actually utilized for what they meant to be, are a lot of work. And with a lot of work comes a lot of wear and tear on bikes. This video explains how to remove, regrease, and replace your bottom bracket. Line your Allen keys (Allen wrenches) up in a row like ducks. An 8,5, and a 4 millimeter will do. You won’t get much done without a rubber mallet, torque wrench, some brake cleaner, and paper towels.
Remove the left crank with an 8 mil Allen key. Your chain gate is next on the list for removal. You may find a lot of dirt and grime in the crank cap. Remove it, clean and grease, it, then replace it. Put everything back to together and you’re good to go. Check out the video. The gentleman may be a bit hard to understand, but he knows his craft.