Just look at an ASCII table - it will tell you the code values for each ASCII character. It is then a trivial matter to conver the code value to the corresponding numerical value of the digit.
All you then need to do is account for the place value of the digit.

Hint: if your string has only decimal digits 0-9, it's really easy; if it also has hex digits A-F, you just have to determine if you're in the 0-9 or A-F range, then it's easy again.

Of course, if you're using 'C', there are standard library function to do it all for you - RTFM.