Just follow the guide and you should be fine with a few caveats. One of the steps is to separate a HA pair to standalone before upgrading anyway so you're halfway there already.
Note that you can't upgrade to BIG-IQ 6.0.0 though as that's for fresh installs only, the latest release is 5.4.0.
Depending on your current version you may need to upgrade to 5.0 first (if running a lower than 5.x version) before you can go to 5.4
https://support.f5.com/kb/en-us/products/big-iq-centralized-mgmt/manuals/product/bigiq-central-mgmt-upgrading-v-5-0-to-5-4-0/1.htmlguid-c4ed65e4-a1ff-47d4-82a6-949c40fb71ec
5.x also has a higher CPU and RAM requirement than 4.x so check your hardware specs, I was prompted to increase to at least 16gb RAM via an onscreen popup.