How to increase the value of a column in a table by a constant by consulting another table?
The UPDATE syntax differs a lot from DBMS to DBMS, especially when there are joins. For Oracle this would work:

update crs_regd r
set marks = marks + 5
where marks &lt; 50
  and exists
      ( select 1
        from crs_offrd o
        where r. crs_cd = o. crs_code
          and o. crs_name = 'DBMS'
      ) ;