So when you say the "user modifies" you are talking about an UPDATE.
DEFAULT values are relevant to INSERT statements only.
It is true you can accomplish whatever automagical UPDATE change you want via triggers but as a beginner, I would squarely advise you away from triggers. They are rarely implemented correctly and make understanding when things go wrong more difficult.
What you want to do is INSERT your row(s) (allowing any DEFAULT values to "happen") and then UPDATE your row(s) with the values you intended.