Display 2 months previous to the input

I need to update the column with the following value, what is the best way to do this. I tried using a case statement with concatenate (+) but when i tried to concatenate, the value gets summed up. I am using SQL Server 2008

Actual Value            Update to

201401                    201311

201402                    201312

201403                    201401

201404                    201402

201405                   201403

.....

201501                  201411

201502                  201412

Sarvan