Is there a way to do Realtime updates to my assistant from my cell phone for example I need to change the first message greeting for Realtime weather updates for the clients who call

You can’t change the assistant’s greeting mid-call on Vapi, but you can update it in real time before the call starts by setting firstMessage with live weather info, or inject messages during the call for updates..

is there an easy way to do it from a cell phone?

Yes, use a mobile REST client or Pipedream to start a call with a custom firstMessage, and optionally inject live updates during the call using the controlUrl..