Antoine,
Yes you still have the inactive partition which is your 6.1.5 in this case..
here is the release note
Step 10
Perform one of the following procedures:
a.
To install the upgrade and automatically reboot to the upgraded partition, choose Reboot to upgraded partition. The system restarts and is running the upgraded software
or
b.
To install the upgrade and manually reboot to the upgraded partition at a later time, do the following steps:
1.
Choose Do not reboot after upgrade.
2.
Click Next.
3.
The Upgrade Status window displays the Upgrade log.
4.
When the installation completes, click Finish.
5.
To restart the system and activate the upgrade, choose Settings > Version; then, click Switch Version.
The system restarts, running the upgraded software
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm