SELECT REPLACE(Address2,'P.O. Box','Post Office Box') AS Address2 FROM CLAddress

https://msdn.microsoft.com/en-CA/library/ms186862(v=sql.100).aspx

better yet...

https://msdn.microsoft.com/en-us/library/ms181984(v=sql.100).aspx

I realize you may not have known about the REPLACE function but the previous posters were trying to push you in the right direction. It's good homework to look at the second link and know what (T)SQL can do with strings.