One may not "return" a loan. Rather, one may pay off the loan.
For most loans, a number or address is provided in order to get pay
off information (there will always be some excess interest that
must be paid beyond the principal). Once one knows exactly how much
is needed to close the loan, one would either write a check and
send that check to the company providing the loan or one would wire
transfer the funds to the company providing the loan.
For payday loans, you will be expected to pay both the fees and
the entire loan as soon as you accept and sign for the loan.