Excel manages time as fractions of a 24-hour day. So, divide the number of minutes entered by 24*60 (1440), and subtract that from the original h:mm, then format the result cell as h:mm.
Assume your h:mm cell you're subtracting from is in A2 and the number of minutes to subtract from it is in B2, with the answer in C2. Format C2 as h:mm, then enter the following formula into C2:
Hope this helps,Code:=A2-(B2/1440)