As a business owner I know it should be a big button on every page. I just spent several min going through links to find a way to upgrade my membership as I use and benefit from this forum daily. Never found a link. I'm sure you can point me in the right direction quickly but it certainly shouldn't be this difficult as to have to hunt for it. A big button? It's not obvious. You're losing money through lost interest.
BTW... Please take this as constructive and with affection for the forum.