In excel you can create a table with the grade points in one column (in assending order) and the letter grade in another column.
use a formula like this:
=LOOKUP(A2,Grade)
where "A2" is the cell referance with the numerical grade points and "Grade" is an array table.
This will convert numbers to letters. If what you want is the other way around, you can't use LOOKUP because you need numeric values, but you can use:
=VLOOKUP(D2,Num,2,0)
where "D2" is a letter grade cell referance, such as "C+" or "B-", "Num" is an array, "2" says the answer is in the 2nd column and "0" is exact match. As before, the array table must be in assending order. There needs to be a value assigned to every possible letter grade.