To update the standard message being sent to clients to Notify them of your Staff's arrival, you will need to add a new SMS Template with the name of "Navigate to Job Template".
The message body can be whatever you would like, however below you are providing the stock example:
Hello {job.contact_first},This is a courtesy message to let you know {staff.first} from [vendor.name] is on the way {and_will_be_arriving_in_approximately_x_minutes}. Please call {staff.mobile} if you are unable to be at the premises at this time.
If you are using Track My Arrival, you can add something like this:
You can track my arrival here: {and_will_be_arriving_in_approximately_x_minutes}