Read Galatians 3:15 Using Other Translations
Brethren, I speak after the manner of men; Though it be but a man's covenant, yet if it be confirmed, no man disannulleth, or addeth thereto.
To give a human example, brothers:even with a man-made covenant, no one annuls it or adds to it once it has been ratified.
Dear brothers and sisters, here’s an example from everyday life. Just as no one can set aside or amend an irrevocable agreement, so it is in this case.