Any simple idea to update a column using update query

Hi friends,

any simple idea to update a column (datatype is varchar) using update query:
(note: i need to do using one single update query (NOT USING STORED PROCEDURE))
(i have to apply in mysql database)

existing data is: "555TST"
have to update as: "555PPP"

(i.e: last 3letter shld be changed)
note:number 555 is a running number may go 99999....
and last 3letters is a fixed to 3 letters only

i am trying as:
select Replace("555TST",CONCAT(LEFT("555TST",LENGTH("555TST")-3),"PPP"));

any nice and simple idea.
NOTE: I NEED TO UPDATE THE COLUMN USING UPDATE QUERY