Planning a route for later? You may well be getting your wheels rolling somewhere else!
You can change the start destination on the route planning screen at the beginning or end of planning your journey.
If you have already planned your journey but want to start it somewhere else, just tap on the blue 'starting point' marker and then press and hold the black tab to move it around, or type your starting point into the 'start' box at the top of the screen.
Otherwise, just tap on the 'starting point' marker and move it around or type a new starting point in before you plan your journey. This is the easier way of doing things if you are planning a route that starts far away from where you are right now!