Hosted Pages and upgrade subscriptions

Hosted Pages and upgrade subscriptions

Hello,

I use "Hosted Pages" in order to subscribe to 1 plan.
I wan't upgrade plan for 1 user.

What-is the good way for this?

If a use same email address I have one error or a second "subscriptions".
I just wan't update the first subscriptions to another.

Can you help me?

Best regards,
Pierre