This article describes how you can update an existing precreated Debian or Ubuntu template.
# vzlist
# vzctl enter [CTID]
# apt-get update && apt-get upgrade
# apt-get clean && apt-get autoclean
# exit
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.