From this article by AppleInsider’s Mikey Campbell, titled Disgruntled HomePod owners say firmware update alters sound quality:

A number of HomePod owners are airing their dissatisfaction over Apple’s first firmware update for the device, saying the new software tweaks the speaker’s sonic fingerprint, a specific sound for which customers paid a hefty premium.

I’ve seen similar comments. Sounds like Apple may have tweaked the EQ settings to emphasize the midrange. That said, I’ve been listening to my HomePod heavily since i did the update (more on that in a sec) and I have to say, still sounds clean to me. When I listen to cleanly produced content, I hear the same separation as always, though I do think it’s possible the bass is backed off just a bit.

To me, the answer to this is, give people access to EQ controls or, at least, a set of presets. No single setting will work for everyone.

As to updating your HomePod, make sure your paired iOS device is updated to 11.3. Once that’s done, you can simply wait, as the default setting has your HomePod software updated automatically.

If you want to force the update, launch the Home app. If your HomePod icon has some text under the icon showing an update available, just tap it, and install the update. If you don’t see it, back off to the main Home screen and:

• Tap the arrow in the upper left corner
• Scroll down to the Speakers section and tap Software Update

That’s where you’ll see the Install Updates Automatically switch. Going to that page should bring up the update.

Just FYI, my HomePod version is now 11.3 (15E217). I’m assuming that’s a build number in there. If you have a different number, please do ping me.