function replace doesnt exist

Hi

im trying to execute an update command on a postgresql DB table using
pgAdmin II

im using the following statement

UPDATE commandlist SET command = REPLACE (command,'A','B')

commandlist is the table name
command is the column
and i want to change the value A to B

but it's giving me the following error message

an error has occured in pgAdmin II:frmSQLInput.cmdExecute_Click:

Number: -2147467259
Description: Error while executing the query;
ERROR: Function'replace(varchar, unknown, unknown)' does not exist
Unable to identify a function that satisfies the given argument types
You may need to add explicit typecasts