The verb fill means to . . .
fill something in
The phrasal verb fill something in means to to write information in blanks (Br.E.).
Please fill in the form with your name, address, and phone number.
fill something out
The phrasal verb fill something out means to to write information in blanks (N.Amer.).
The form must be filled out in capital letters.
fill something up
The phrasal verb fill something up means to fill to the top.
I always fill the water jug up when it is empty.