Fill a plastic bag halfway with vinegar.
Place the shower head inside the plastic bag and secure it with a zip tie.
Let the shower head soak overnight.
The next morning, remove the bag and scrub the shower head with a toothbrush.
Finally, turn on the faucet and let the water run for a few minutes.
Vinegar is one of the most effective ingredients for removing mineral residue that is the main cause of clogged shower head holes.

To clean a removable shower head:

Remove it from its place and place it under the stream of water, but face down (the holes should be oriented towards the stream of water).
Rinse until any traces of lime and surface dirt are removed.
Soak the shower head in a bowl filled with vinegar overnight.
The next day, scrub with a toothbrush and then rinse as in point 1.
Now that you know how to unclog the holes in your shower head, try this method immediately to get your water pressure back to satisfactory.