Multiply the denominator of the fraction by the integer, and then add the numerator. The result should be the new numerator, over the old denominator. For example, with the mixed number 2 5/8, multiply 8 by 2 to get 16, and then add 5 more to get 21/8. If your number to subtract is 1 3/4, your second improper fraction would be 7/4.
Press the left-parenthesis, or "(" button, on the calculator. Then type in "2," "1," "/" and "8." Then hit the right-parenthesis button, or ")."
Hit the minus button, and then "(," "7," "/," "4" and ")." Then, hit "Enter," "EXE" or whichever button solves problems on the calculator. You'll get a decimal result. For the example, you should get 0.875, which equates to 7/8.