You can Upgrade/Downgrade your hosting plan from your billing client area which is more secure and user-friendly. Here is how you can do this by yourself.
1. Log into Eicra Billing System at https://billing.eicra.com/clientarea.php. See the image below
2. Click on "My Services" link from the top.
3. On the "My Services" page, the signed up hosting accounts are listed here, select on the last icon from your domain row. Here it's shown below
4. You will redirect to the product detail, select on the button named "Upgrade/Downgrade Package".
5. Select your package and select on the button "Click to Continue >>"