Your post is a little unclear to me, but have you tried wiring your Arc temporarily with an ethernet cable? Generally speaking, an error of 30 suggests difficulty or a time out in your Sonos system reaching the update servers properly. This could be due to simple wifi interference, up to some sort of VPN blocking the speakers. Or lots of things in between those two. Your post doesn’t provide much in the way of detail to be sure. 

I’d also recommend that you submit a system diagnostic within 10 minutes of recreating this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

No , I have never try wired since wifi work for me since setup2 years. Only after changed my router that updates seem not responsive well. Will try again before sending a diagnostic report further. Possible a small issue and sound still great with current Ver15.2. Thanks for your advises. 

Checked network was correct and found sub version still at 15.2 from “About system“. Maybe it is reason for error 30. 
 

it means ARC version already updated and awaiting for Sub ? How to reset Sub

Shouldn’t reset the Sub unless Sonos tells you to.

You can power cycle it by unplugging power for 30 seconds.

Not sure if it would help but you could try a temporary Ethernet cable to the Sub for the update.

Like to try this action. Can give me some detail for this procedure? It is require to unplug from socket or can unplug from sub power cable as current ARC and Sub plugged into extension cable ? Looking at those hidden wires made me frightened already.

Unplugging from the wall socket or extension cord is usually the easiest option.

I picked up a cheap 50 foot Ethernet cable for making temporary connections here, nothing fancy needed.

50 foot and other lengths: (other sellers may have better prices, don’t get ripped off for a $100 super-special cable!)

Flat so it stores well: https://www.amazon.com/Cat-Ethernet-Cable-White-Connectors/dp/B00WD017GQ/ref=sr_1_4?th=1

Good enough: https://www.amazon.com/AmazonBasics-RJ45-Cat-6-Ethernet-Patch-Cable-Feet-15-2-Meters/dp/B0134QJH4G/ref=sr_1_3

Somewhere around, I have one of those, perhaps from Fry’s before they went out of business….but I’ve not needed it since I set up reserved IP addresses for most of my network devices in my router. Which is probably why my mind can’t think of where it is now. 

Not a solution for your problem, but to make you understand the Sonos system better: the app is just a remote ro the system. The system does not need it to update  - it should find the updates itslef. You phrase “it is able to finally update with existing Ipad” does not represent how updating works on Sonos.

Finally solved my Sub upgrade issue by good suggestion to unplug from power and press infinite button. 
 

After that add back to connect to my ARC. 
Now everything back to its original and sound still as great. Thanks for all members suggestion too. 
Have a great day ahead. 
 

Watching my favourite Netflix Fubar.