Arm yourself with a decent tuner and a screw driver.
Check the open string tuning and also at the twelve fret and the 12th harmonic.
If you get a flat reading at the 12th fret compared to the 12th harmonic, slacken the string, then turn your adjusting screw anti clockwise.
Sometime you also have to gently ease the saddle back/forward by hand as well as turning the screw.
Take up the slack retune.
Repeat for all strings.
If you have had to make big adjustments then it may be that the action will need another tweak.