How To Optionally Update Field In Stored Proc?

Hello,,,I need to update table but not all fields in stored proc have to be with value (some of fields will be NULL), so i need to avoid updating any field that his parameter with null value.I need the syntax for (IF) statement to optionally update that field.something like this :CREATE PROCEDURE [dbo].[MyUpdate]
(
@ID int,
@Field1 nvarchar(50),
@Field2 nvarchar(50)=null,
@Field3 nvarchar(50)=null,
@Field4 nvarchar(50)
)
AS

UPDATE MyTable
SET Field1 = @Field1,
if (@Field2<>null) Field2 = @Field2,
if (@Field2<>null) Field3 = @Field3,
Field4 = @Field4
WHERE ID = @ID

 I saw example before but i can't remember where.

Thank you in advance