1. Create a container based on the template you want to update. for this you can use the GUI.
2. Then enter the asked values and start your container.

1. Login in the Proxmox node where the VPS was created.
2. Check your container's ID (check CTID column) with the following command:

   # vzlist

3. Enter the container:

   # vzctl enter [CTID]

4. Configure the repositories.
5. Update metadata and install updates:

   # apt-get update && apt-get upgrade

6. Cleanup:

   # apt-get clean && apt-get autoclean

7. Clean unused log files, .bash_history etc, then exit from a container:

   # exit

8. Stop a container.

To complete this task you can create a new tarball:

# cd /var/lib/vz/private/[CTID]
# tar --numeric-owner -czf /var/lib/vz/template/cache/debian-7.8-mininal-i386.tar.gz .

Where [CTID] is the container's ID.

Another way is backup your container using the GUI and move the backup file to /var/lib/vz/template/cache/. Remember select the compression method: GZIP (good) and Mode: stop.