You update data with the SQL UPDATE statement. When you update data and you only want to update a single row, make sure you have a filter on your query of exactly what you are updating, otherwise you are going to wind up updating the entire table.
UPDATE --the table you want to update
SET --the new value in the column you want to change
WHERE --The conditions you specify
Simple Update Example
DECLARE @Person AS TABLE (ID INT, FName NVARCHAR(100), LNAME NVARCHAR(100))