need to upgrade f5 ltm from 11.4.1HF5 to 11.5.1 HF7

there isn't a true document except from a small section in the release notes

http://support.f5.com/kb/en-us/products/big-ip_ltm/releasenotes/product/relnote-ltm-11-5-1.html

section Installation overview, which points to the general active-standby / active-active upgrade documentation which in my opinion still seems to focus on 10 to 11 upgrades and not the 11 to 11 upgrades.

https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-upgrade-active-standby-11-3-0.html

beyond that you should read the release notes and check for any possible issues and tackle those before the upgrade.