how to update value from 1 to 0 and 0 to 1 in sqlserver

UPDATE dbo.Test SET ColB= 

                                           (CASE ColB WHEN 1 THEN 0 ELSE 1 

                                          END);

 If the values are always 0 or 1  

Then you can use this also

UPDATE dbo.Table1 SET ColB= (ColB^ 1);

^ is bitwise or (exclusive or)