If I understand correctly the following commands should upgrade your 7.6 system to the latest NS version (7.9.2009).
Upgrades across multiple versions is not a tested procedure: the system should be updated periodically.
Before proceeding, read the release notes
yum install -y http://mirror.nethserver.org/nethserver/nethserver-release-7.rpm
signal-event software-repos-save
yum update -y