Why would you want to update all records with AGE = 20 to same LAST_NAME?
If thats what you really want to do, then you don't require a CURSOR, just use the Update.
UPDATE TBL_EMP SET LAST_NAME='kanjan' WHERE AGE = 20;
It will update all rows that have AGE = 20 with the LAST_NAME you specified.
If you want to use CURSOR for UPDATE, you still need to DECLARE, OPEN, FETCH that cursor before you can UPDATE any row. Once you FETCH a row then you can decide if you want to UPDATE that record or skip it and FETCH the next record.
But it is best to have the SELECTION criteria in the WHERE clause of the SELECT, so that you might not be locking up the rows that are not really needed for update.