Using the Proxmox GUI to change resource allocation is the preferred way to adjust the container resource. Any changes made get committed to the container instantly without needing to power-cycle it. For day-to-day operations, the GUI provides almost all the resource options to be changed with a few clicks. 

To change a particular resource, we need to select a container from the left-hand navigation bar, and then we need to select the resource to be changed. For example, if you want to increase the allocated CPU cores to 2 from 1, you need to select the Cores line item and then click on Edit to open the CPU Core dialog box. The following screenshot shows the Resources currently allocated to the example container #101:

To increase allocated storage space, we need to select the Root Disk line item under Resources and then click on the Resize disk button to open the dialog box: 

As of Proxmox 5.0, we can only increase the size of the allocated storage but cannot decrease it. We can type in a value in GB or use the up and down arrows to adjust size. It is important to note here that the value we will select here is not the total size of the allocated space. This value adds on top of the already allocated space. For example, in our example container #101, the allocated space is currently at 4 GB. So if we want to increase that to a total size of 6 GB, we will type in 2 in the dialog box, which will increase the size by 2 GB. Click on the Resize disk button in the dialog box to finalize the value.

We can verify that the disk space has indeed increased by running the #df -H command from inside the container. The following screenshot shows the command output, which shows that the size of the root mount point has increased to 6.3 GB: