• First you resize the VM disk offline on Proxmox
• Boot the VM and enter the following in a shell:

sudo su
 
# Check sda or vda or any other disk you wish to resize
fdisk -l /dev/sda
 
# There will be a red message mentioning the resize
# Use parted now so it asks for a fix
parted /dev/sda
  > print
  > Answer: Fix
  > resize part 3 100%
  > quit
 
# Next you resize the physical volume of the last partition if available
pvresize /dev/sda3
# Then you resize the logical volume to occupy the rest of the drive
lvresize --extents +100%FREE --resizefs /dev/mapper/ubuntu--vg-ubuntu--lv

• Take a breath, it's done.
• I should read up on LVM management.