I'm not sure about graphing calculators in general, but on the site calculator if you want to convert an improper fraction to a mixed number here's a procedure.....
Input a/ b and hit =
This gives : Whole number and decimal (1)
Take this result and subtract the whole number part......it will give you the remaining fraction (2)
So....the result will be the whole number part in (1) and the fraction in (2)
78 / 23 = = 3.391304347826087 (1)
3.391304347826087 - 3 = 9/23 (2)
Result 3 9/23