Depending on where you are seeing it, there are 2 places to change this.
You can change it in your self care page on the aprs id for that node.
Changing it in the callsign area of you mmdvm.ini will affect another area.
just be aware, when you edit the call to a non-conventional call in the callsign of the .ini, the node will no longer show up in the “my hotspots” area on your BM dashboard. It will still work, but you won’t be able to see it until you put just your callsign back in.
I have mine set to 975 Bridge
on BM it shows up as 975 Bridge in last heard.
That is my “call sign”
in aprs I have it set to Analog as call sign.
each one affects a different column in last heard.