When you unloaded the 2.85mm filament, the LCD should show "Remove Material" and two options, "Ready" and "Cancel".
Press the dial to select Ready. It'll then give you a message to insert the new material. "Instert new material from the backside of your machine above the arrow".
Press the dial to select "Ready". Now Insert the 1.75mm filament into the opening near the bottom of the feeder. The motor will slowly feed the filament through the guide tube.
Once you see the filament pass through the feeder and inside the guide tube, press the dialer to select "Ready" - It'll then increase the speed of the feeder and push it through the extruder. The LCD will show "Forwarding material" - Give this a moment to purge some filament.
The LCD will show "Wait till material comes out the nozzle". Once purged, select "Ready" using the dialer. Now you can exit this section and return to the main menu.
You are now safe to print!