upgrade from 10.2 to 11.6

You'll find some good information here: https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-upgrade-active-standby-11-6-0/1.htmlunique_42999993

Without seeing the current status of the device you upgraded, it would seem like you are probably at this point: "With the new version software installed on Device B and traffic-group-1 in standby state, you can force Device A to offline mode, changing Device B to active state so that it can pass traffic, and then install the new software version onto Device A. When installation of the new version software onto Device A completes, you can reboot Device A to the location of the new version software image."