You would essentially "vote" with which fork you were choosing to acknowledge with your own node, should you be running one.

Also I'm fairly certain a majority of hashpower, not just users would need to be on board with the change.

You could argue that as the primary beneficiaries of such a change (increase cap => increased distribution to miners) they would be for it, but I think most rational actors see the catch 22 of trying to profit by removing one of the core attributes that makes bitcoin valuable (it's scarcity)

Bitcoin works by consensus though. Nobody can be in that position. the best you can do is make a fork. Those who don't agree with the rule change will keep mining with the old rules in place