see user guides

https://www.Meridian-audio.info/public/861user%5b853%5d.pdf

https://www.Meridian-audio.info/public/Meridian-config-guide%5b1081%5d.pdf

The process should be similar for my G68 and your 861, I used USB 2.0 To RS232 Serial DB9 9 Pin Converter PC Computer Adapter Cable and a serial nul modem cable to connect computer to hardware, this should be configured to coms 1 on the pc.
https://www.eBay.co.uk/itm/364016035472

the wiki on here is very useful
https://www.meridianunplugged.com/wiki/index.php?title=Meridian_Room_Correction_Tips_and_Tricks

Hope this helps
Rgds
Stuart

Linn LP12, Ittok LV11, Arkiv, Linn Kairn as a phono stage, MPmax 221, DSP6000's, WiiM Pro streamer.