When creating a function dynamically, it is a best practice to perform a check if a particular function you want to create has already existed in a database. To perform a check of an existing function you can use the following EXISTS keyword. If the function has already existed then you have to drop the function first and then perform a creation of the function.

IF EXISTS (SELECT * FROM   sys.objects WHERE  object_id = OBJECT_ID(N'[dbo].[my_function_name]') AND type IN ( N'FN', N'IF', N'TF', N'FS', N'FT' ))
    DROP FUNCTION [dbo].[my_function_name]
GO 

CREATE FUNCTION [dbo].[my_function_name](
       @ID NUMERIC(18,0)
    )
    RETURNS BIT
AS
    BEGIN
            DECLARE @NoOfRecords INT = 0
            SELECT @NoOfRecords = COUNT(*)
            FROM [dbo].ExampleTable
            WHERE ID = @ID

            IF @NoOfRecords > 0
                 BEGIN
                     RETURN 1
                 END 

            RETURN 0
    END

GO 

If you need to perform an update of a function, the way to check is pretty much the same as checking if a function already exists. You can re-drop the existing function and recreate it again..