First, you will need to login to my.webo.hosting.
In the main menu click on Services then My Services tab as shown on the picture below.
Select Cloud service that you would like to upgrade and click on the name of the service.
In the left menu look for Change Plan Options and click on it.
This will open a new section where you can configure your product cloud upgrade options.
In the field, Disk Space enter the new desired amount in GB and click Continue.
Eg.: If your cloud has 100GB of disk space and you would like to add 20GB additional space, please enter 120GB.
Please note you will always have to add to the existing amount.