You can upgrade your VPS in the "Upgrade" tab of your VPS tab in your control panel
You can choose if you want to upgrade only the CPU and RAM or also the disk size.
If you choose to upgrade the disk size as well, it will not be possible to downgrade to a smaller disk size in the future.
Please make sure you have enough credit on your wallet to pay for the plan difference.
After your server is upgraded you next billing term will be at the new plan price.
If you need to downgrade in the future you will need to open a ticket as this is not yet automated.