When new software is available for the HomePod it is installed through the Home app on iOS. VoiceOver users are able to access the notification in the Home app of there being an update, but the button to install the update is not accessible.
To be able to reproduce this issue there will need to be a HomePod software update available.
Assuming there is, open the Home app on the iPhone used to manage the HomePod.
Near to the top of the resulting page you should find a notification that a HomePod software update is available.
When speaking this announcement, VoiceOver will speak that there is a button to install the update, but there is no way to directly access and activate that button.
It's been reported that enabling Screen Recognition can make the button accessible.
If this doesn't work for you, it may be necessary to get sighted help with installing the software update. If this is not an option, or if you simply want to try and do this yourself, the button is visually located towards the top right corner of the screen just below the top menu bar.
If you are able to locate and activate this button with VoiceOver disabled, there will now be a message near to the top of the screen saying that the download is in progress, and this message is accessible with VoiceOver once it is re-enabled.
Comments
Temporary Workaround
I too noticed this issue when trying to update my various HomePods to 14.3 but have figured out a temporary workaround.
When in the Home app and on the screen with the Update button, temporarily enable Screen Recognition and the Update button is accessible to VoiceOver.
After tapping on update, then you can turn Screen Recognition back off.
This worked for me when updating to 14.3 and again yesterday when updating to 14.4.
HTH!