Try loosening your showerhead with your hands by twisting it counterclockwise. If this does not work, you can try using a wrench or the vinegar method below!
1. fill a plastic bag with white vinegar.
2. wrap the bag around the showerhead so that it's completely submerged
3. secure the bag to the shower arm with rubber bands
4. wait a few hours to let the vinager descale the buildup on the showerhead
5. remove the bag and twist off the showerhead counterclockwise by hand or wrench