If the headset is old school loose or caged bearings, disassemble, clean and reassemble with new grease. If the bearings are sealed cartridge type, remove the cartridge, carefully pry up the seal, clean and regrease. Headset bearings are for low speed support rather than high speed so a degree of wear is acceptable. If in doubt just replace loose balls or an entire cartridge. Cartridge ID is usually on the side or take to a shop for measuring. For grease use Phil’s, marine grease or my choice (just for headsets) - heavier moly. Likely plenty of DIY’s on youtube if needed. For loose bearings, a magnet is your best friend.