Unscrew and remove the showerhead from its holder.
Pour vinegar into a bag big enough to cover the showerhead.
Wrap a rubber band or zip tie around the submerged showerhead. If you need a thorough clean, leave the showerhead underwater overnight
After soaking the head in vinegar to release deposits and dirt, remove the bag and clean with a plastic scrub brush (or a spare toothbrush).
Rinse the showerhead to eliminate vinegar residue. Vinegar is a good cleaning remedy for showerhead buildup, limescale, and mould.
If there's too much mould overgrowth to remove with vinegar and a brush, you may need to use Tilex Mold & Mildew ($15) as a last step.