EDIT: I think I duplicated Recyan :banghead:
I think this is what you want. Adjust your expression according to your table, fields and grade ranges.
SELECT tblGrades.Grade,
IIf([grade]>=90,"A",
IIf([grade]>=80 AND [grade]<90,"B",
IIF([grade]>=70 AND [grade]<80,"C",
IIF([grade]>=60 AND...