If you're finding that your site is a little more or less popular than you thought, or you need more or fewer seats, you may need to change plans. All you do is go to account >> plan & usage and then click the change plans button in the top right:
Once you are on this page, it's all fairly straight-forward to change plans, and keep in mind any past payments will automatically be pro-rated and applied to your new plan if you have a credit. In other words, if you paid for a certain period (month or year) and haven't used that up, Ethnio will automatically reduce the amount you have to pay for any upgrade.
If you need to switch to free or cancel, follow the instructions here to do that - it should only take a few seconds.