Firmware build number can be found after pressing the globe button I mentioned just now, then choose the node you want see. Then you will find the small text at the middle bottom when you scroll down.
Login to your main router, and find the wifi settings at the router.

Or you can do like what others say. Set Deco as router instead. That will be better.

Ok. Deco version: 1.4.2 Build 20200727 Rel. 37987

Ok, will set it up as router. Do i need to reset both AP set before restarting? Just go to the Deco app and remove both and then add again?

Yes. Remove the node first, then only remove main deco.

Ok, recommended to update the firmware?

Till now, the devices is still not smart enough to change AP. 

The older devices are not smart enough to change AP. My suggestion is to split the band to 5Ghz and 2.4Ghz using different SSID by using the guest network. Just connect those that are capable of 5Ghz to the respective band.

You can upgrade the firmware if you have Apple devices that uses iOS 14.

Ah ok. So, for example, main WIFI network set to 5Ghz, and create a separate Guest network that used 2.4Ghz - both using diff SSID.

As for firmware, so if I only have Google Android devices, recommended not to update?

Yes.
I think is better to update it, cause in 1.4.3, they now support WPA-2 AES encryption which is safer compare to 1.4.2.