Now that your new filament is at the nozzle, it’s time to feed it through the nozzle with the extruder. But, before you can start using the new filament you must purge the old filament from the nozzle. This will ensure that the old filament is gone from the hot end and you will be printing only from the new filament.
Note: If you are changing from a higher temperature material to a lower temperature material, you should set the nozzle temperature to the higher value until the purging is complete.
The Ender 3 has some features in its software to help purge the old filament. Click the control knob to open the settings menu and scroll to Move Axis -> 1 mm -> Nozzle -> 15 to 20.
The extruder will start feeding material through the nozzle, which will purge the old filament and feed in the new filament. You may want to do this more than once to ensure all of the old filament is out of the nozzle.
The nozzle is purged once you see the new color or material flow out the nozzle.