Q.Which should be upgraded first BIOS, BMC or Host Boot Firmware?
A. Please find below the upgrade order for all Nutanix firmware and software components. For hardware that has SATADOM, SATADOM should be upgraded first before the rest of the components. Always update BCM before the BIOS.
Acropolis Upgrade Guide v5.17

Q. Can numerous upgrades be performed at the same time? For example, can I upgrade Boot Firmware on all five hosts in the cluster at once?
A. You can select all components for all hosts to be upgraded. However, the upgrade will be done on each component of a host one by one. So only one component of one host will be upgraded at a time. When completed the LCM will proceed with the upgrade of the next component and so on. The non-critical and non-impacting components are upgraded first.

Q. Should these upgrades be performed after hours? Is there an estimated timeframe for how long these upgrades take?
A. No, the LCM upgrades are non-disruptive, so it does not require downtime.
The estimated time for the upgrade of one component on one host is around 30-45 minutes.

Please check below the LCM guide for more information about how LCM upgrades work:
Life Cycle Manager Guide v2.3

Also, please check the below KB article for the pre-requisites and most asked questions about LCM upgrades:
KB-7536 LCM-Generic FAQs