what part needs to be replaced on my GE dryer?

my general electric dryer, model#DBXR463EB1CC,This dryer is only 3 years old,and the drying mechanism isn't working correctly.It takes about 3 cycles until the clothes are dry and by this time the clothe stink from not being dryed fast enough,which are about 40 min. each cycle. I've tried each section on the knob low tumble etc. and nothing.please let me know what part needs to be replaced or procedure I need to take so i don't have to pay for somebody to come out.thanks