This is done using the REPLACE function
To strip out "somestring" from "SomeColumn" in "SomeTable" in the SELECT query:
SELECT REPLACE([SomeColumn],'somestring','') AS [SomeColumn] FROM [SomeTable]
To update the table and strip out "somestring" from "SomeColumn" in "SomeTable"
UPDATE [SomeTable] SET [SomeColumn] = REPLACE([SomeColumn], 'somestring', '')