How do you upgrade a WPMU plugin?

I cannot upgrade any of your plugins the normal way. Is there a trick to this, and if so, would you point me to it? I’ve done a search of your site and if the answer is here, it’s well and truly hidden. It is terribly frustrating to pay for premium plugins that cannot be easily updated to newer versions. Someone told me they do it by diving into the files through the cpanel to do it, but this seems impractical. Tell me there is a simple way to do this. Also, a suggestion: Put that information where it can easily be found. Thanks.