You can't. The premium badge under mine and other's names are not related to our in game rank. For example, I am an Admin in game. The premium forum rank is bought separately from the ranks in game.
The ability to change it is a perk that comes with premium. As a normal forums user, that will change depending on that amount of posts you have. For example, over 500 posts it will change to #GhostArmy
Posts are comments. For every comment you put on a thread your post count goes up by one. Please don't spam comments though.
You can either post messages on threads (without spamming) and work your way up to Dedicated Minetimer or donate for Premium on the forums! More information here
If you buy something from Minetime and don't get it, you should email [email protected] and send her the receipt.
You can apply to be helper. If you are accepted, your under your name "Member" sign will become "Helper" sign.