Recrystallize the mixture from hot water.
The solubility of acetanilide in hot water is 5.5 g/100 mL at 100 °C
and 0.53 g/100 mL at 0 °C.
Urea is highly soluble in water (108 g/mL at 20 °C).
Assume that you have a mixture containing 5.5 g of acetanilide and 5.5 g of urea.
Boil the mixture with 100 g of distilled water and perhaps some activated charcoal and filter through a fluted filter paper.
Cool the filtrate in an ice bath to 0 °C.
The urea will remain in solution, while beautiful colourless plate-like crystals of acetanilide will precipitate out.
You can then remove the pure crystals by suction filtration.