First, open Proxmox UI in the browser: (https://192.168.1.x:8006)
Click on the container to resize and stop it. Note the number of the container.
Then, click on "PVE" and then on "Shell".
Type the following command: pct resize 10X rootfs XXG
so, if we want the contains 102 to be resized from 10G to 40G, we type: pct resize 102 rootfs 40G