REPLACE Integers With Text Data

I am working with a database named €œDocuments€? that contains 4 categories of text documents, each having its own number designation in an integer datatype column named SectionTypeId:

1 = Text
2 = Report
3 = Background
4 = Index

I would like to create a new column named €œDocType€? in which the integer data type for each document is replaced with a varchar data type letter (1 = T, 2 = R, 3 = B, 4 = I). I was able to easily create the new column and cast the data type from integer to varchar:

--CREATE NEW COLUMN €œDocType€? WITH VARCHAR DATATYPE

ALTER TABLE FullDocuments ADD DocType VARCHAR(1) NULL
Go

--UPDATE NEW COLUMN WITH CAST STRING

UPDATE FullDocuments SET DocType = CAST(SectionTypeID AS VARCHAR(1))
Go

But I have problems with the REPLACE method for replacing the numbers with letters. First I tried this based on the examples in MSDN Library:

--REPLACE NUMBERS WITH LETTERS

UPDATE Fulldocuments REPLACE (DocType,"1","T")

Which produced an error message: €œIncorrect syntax near 'REPLACE'.€?

Thinking that the datatype may be the problem, I tried this to convert to DT_WSTR data type prior to replace:

UPDATE Fulldocuments REPLACE ((DT_WSTR,1)DocType,"1","T")

Which produced the same error message: €œIncorrect syntax near 'REPLACE'.€?

I have never done a REPLACE before, so any suggestions for accomplishing this would be appreciated.