Update SQL Value based on another column/field

I can do
UPDATE Tblx
SET Column1 = '1'
WHERE Column2 = 'xyz'
But what I want to do is:
Get value of a column and then add say 30 days and save this into another column. There are only 6 values so I don't even need to have a variable for the item to look for, I can run the code 6 times but not sure what to look for in order to code this in SQL.
I could do it in .NET but it would be better to know how to do it in SQL.