Grip the aerator with your hand and unthread it to the left to remove it from the end of the faucet spout being careful not to drop any parts down the drain

.
Disassemble the aerator, using a toothpick or paperclip, taking note of how the parts go together. Rinse off any large pieces of sediment.

Soak the screen and other aerator parts (as needed) in vinegar to dissolve mineral deposits (scale).

Rinse, reassemble, and reattach the aerator.