How Do Members Upgrade their Accounts?

Hi,

I think this might be apparent to everybody else, but I cant seem to find a button where users can click on upgrade their account.

How do I create a Upgrade Button that once clicked, allows my users to see what levels they are at, and then choose the levels that they would like to upgrade to?

I've manually created a Page with the short code [subscriptionform], but if I am a user logged in, all i see is the message: "You are already a member of our site. Thank you very much for your support".

So I am really baffled about how I can enable members from a free subscription level to upgrade to a paid one.

Please help!

Thanks so much!

Cheers
Jiahui