I have around 4000 records, and I have to update a custom field (e.g Rating) in Account object, where condition is that if value in the field is 10 update/change to "A" and if the value in a field is 20 change to "B" in all the records.

what is the best way i.e SOQL or data (Data Loader), I am new to SOQL

Thank you very much