Hi,
Since i am not clear about your 2 table from which you want to update data in GRADE_CODE of PR_GRADE_MASTER table. Kindly replace the required table name / column name / where conditions in below query.
update PR_GRADE_MASTER
set
GRADE_CODE='Your 2nd Table.Required Col'
from
'Your 2nd Table'
where
'Matching cols conditions'
Hope this will help you.