Right from their great article

  1. Login to your Google Compute Engine
  2. Navigate to Disks
  3. Select your disk
  4. Select Edit
  5. Input the size and save
  6. (ext4) While the partition is mounted, run the below command
  7. resize2fs /dev/[DISK_ID]

I found this article from Stackoverflow about having trouble using resize2fs. One has to resize the actual partition and then resize2fs:

  1. growpart /dev/sdb 1
  2. resize2fs /dev/sdb1