Open your vm

The resize is not available when the vm is ON

Before changing hardware configuration you need to turn of the VM

You can turn off by using SSH or the web interface :

Turn off option :

The status is now powering off :

Press the refresh button to refresh the status until the status is OFF :

Now you can click on Resize button to add or remove resources :

Update the values :

And click on Resize

Now just power on the VM by pressing the arrow button