It can't be adjusted just need to replace the seal on the ballcock valve ( about 10 dollars at your supply house) we like the black replacement and don't like the pink one so make sure to get the black one.
Unscrew the rod and ball from valve arm, take bolt out that hold the arm to the assembly, pull arm out and away from assembly.
Reach in and pull plunger from assembly, remove old worn/damaged seal and install new seal.
Install in reverse of above instructions.