First, you need a good reason for doing so:
- The decline reason is no longer valid
- You have a compelling counter-argument against the decline reason.
Either way, you can edit the post/add an answer explaining whichever of the above two points is applicable.
Additionally, you may go into the post history, find the declining mod, and @notify them (you can @notify anyone in the post history), asking for removal of the tag. You can alternatively flag, though I personally prefer the previous option.