First thing to do is to disassemble the phone to take out the scroll part you are going to clean
After you take out and disassemble all the components of the scroll, take a toothbrush and and put an alcohol or cleaning fluid then brush every parts of the scroll to remove the dirt's accumulation
After cleaning the parts assemble it back, be sure to put the rollers and the ball into its original position ( if not in proper position the UP direction will be the LEFT and the DOWN scroll will be the RIGHT and vice versa ) if this happen just reverse the position of the roller
Scroll after cleaning and already put back into place
Assemble the phone and test. ENJOY