muss up
To cause something to become disheveled or untidy, especially one's hair. A noun or pronoun can be used between "muss" and "up." I just spent an hour getting my hair exactly how I want it, so don't go mussing it up like that! Please don't muss up your hair before you get your pictures taken for school.
muss someone or something up
to put someone or something into disarray. Don't muss me up! You mussed up my hair. I'm afraid I mussed myself up a little.