7.8 DOC contains that information https://access.redhat.com/documentation/en-us/red_hat_fuse/7.8/html-single/installing_and_operating_fuse_online_on_openshift_container_platform/index#upgrade-on-ocp-script_manage

however, 7.9 DOC contains only the information that when users installed Fuse Online via install script, they can upgrade via OperatorHub (which is true only for OCP 4.x) but it is not able for OCP 3.11. We need there a section for 3.11 too. ( the steps are the same as in the 7.8 DOC)