When there are more than two outcomes, use IFS. It takes pairs of test and result, and returns the result for the first test that's TRUE:
=IFS(B2>=90,"A",B2>=80,"B",B2>=70,"C",TRUE,"F")
Order matters. A score of 95 is also above 70, so the highest band must come first. The last pair, TRUE,"F", is a catch-all: TRUE is always true, so anything that fell through gets F. Without it, IFS shows #N/A when no test matches.
Before IFS existed (it came with Excel 2019), people nested IF inside IF: =IF(B2>=90,"A",IF(B2>=80,"B","C")). You'll still meet nested IFs in older workbooks, and they work in every version. Each inner IF goes in the "false" slot of the one before it.