Close the main valve further upstream of this valve (you don't want water to spray put when you do the repair)

1) Remove handle - there's a small allen screw to unscrew, then push out the handle.

2) korek put this decorative piece.

Unscrew the cartridge inside (anti clockwise). Pull out the cartridge. Clean the cartridge, wipe with silicone grease. Install back if you cannot wait for new cartridge.

Take a photo of the cartridge to buy replacement cartridge. Remember to measure the dimensions (diameter, length, etc - anything that can ensure you buy the proper cartridge).

Replace with new cartridge.