The description of that AVR on the Yamaha website says it support Musicast.
You would intall the Musicast controller app on your phone or tablet to control the Musicast functions of the receiver.
Page 58 of the owners manual tells you how to connect it to the Musicast network if you have other musicast https/usa.yamaha.com/products/audio_visual/av_receivers_amps/rx-a870_u/downloads.html#product-tabs
The only firmware upgrade is unrelated to Musicast.
Perhaps you don't realize that you would need other musicast endpoints to actually set that up.
Good morning, yes thank you I know. It supports the musiccast but I do not believe it supports the "surround musiccast" that uses 2 wireless surround speakers. Only the 2018 models have it and I wanted to know if there were expected firmware updates for 2017 models of the aventage group.
It may require hardware rather than just software. Only Yamaha could tell you.
You can use a pair of stereo bluetooth speaker by connecting a speaker to line level converter to the surround speaker terminals. That will provide an output for a BT transmitter. You have to use a stereo pair rather than two mono BT speakers to get left and right surround channels. The exception to this is the JBL speakers that can be configured as left and right in the JBL Connect+ app.