If you find your headset is too loose or too tight you need to adjust the bearing(link). Riding with a maladjusted headset will be more difficult and less comfortable and will wear out the bearing prematurely.
Some bike headsets have a knurled ring on the adjustable race, these can be adjusted with a large pair of slip-joint pliers. If it has flats, either a hexagon or two parallel flats, you will need a bike headset spanner of the correct size to turn it. A bike headset spanner is thin so it can turn on the nut’s narrow flats. The lock-nut on top can be turned with a second bike headset spanner but it may be possible to use a large adjustable spanner for this, so you may be able to do the job with only one bike headset spanner.
Once the lock-nut is released the adjustable race can turn. If the bearing is too loose, screw the adjustable race down – clockwise – as far as it will go with your fingers or using a tool gently, then turn it back a quarter turn. If the bearing is too tight, turn it anti-clockwise a quarter turn.
Check the bearing to see if it’s within the target of ‘free to turn’ and ‘not too loose’(link). If it’s too loose or too tight repeat the previous step making finer adjustments.
Once you have the adjustable race in a good position hold it still with a bike headset spanner or big pliers. Screw the locknut down until it jams against the adjustable race to lock the two parts together.
Make a final check that the bearing is in the target zone.