Go on your Profile Page and click on the Change Now! button
You will be presented with all the possible plans, just select the one you wish to use.
In this example we are going from Premium to Elite account:
You will be prompt with the payment page: the full price for the plan is displayed, but you will be billed ONLY for the difference between the current plan and the new one and only for the remaining time of the subscription.
The maths is done once you accept the upgrade, so you won't be displayed here the real value.
How it works?
We have a system that allows you to pay only for what you are using: if your budget increases/decreases at some point during the subscription, you can change it and pay only for the remaining time that you will use with the higher subscription.
Can I upgrade my plan if I already over my plan spent?
Sure. You can upgrade and you will be able to create new campaigns immediately. However, you may need to logout and login again to stop seeing the alerts.