If theres only two values you could create a view with static text.
create view newtable as
select column1, column2, 'EM' column3, column4
from table1
or you could translate it
create view newtable as
select column1, column2,
CASE WHEN column3 = 'PM' then 'EM' else column3 end column3,
column4
from table1