We can subtract the 5bit binary numbers 01101_{2} (13_{10}) and 01001_{2} (9_{10}) by converting 01001_{2} to its negative equivalent in 1's complement and adding this value to 01101_{2}.

01101 (13)  01001  (9) 

01001 > 10110 

01101 (13) + 10110 +(9) 100011 (?) 

00011 + 1 00100 (4) 
Let's review the steps for subtracting x from y with an nbit 1's complement representation.
 Negate x using 1's complement.
 Reverse all the bits in x to form x.
 Add x and y.
 If the sum exceeds n bits, add the extra bit to the result.
 If the sum does not exceed n bits, leave the result as it is.