You can change your myCloud Pro, Standard or Light subscription at any time. Click on "Change subscription" in the settings.
Under Settings you can also see or check which subscription you have at the moment or after the adjustment. Depending on which Swisscom subscription you use, various options are available to you. You can find an overview here.
Please note: If your current storage usage is higher than the storage space available after the switch, there may be restrictions on the use of your account. You can read more about this here.