Before trying to unload the filament manually, make sure that it can't be unloaded following the standard procedure:
If the automatic filament unloading has failed, it can be due to the extruder grinding the filament. In order to unload the filament you can follow these steps:
Go to the temperature menu and heat the hotend to the printing temperature of the loaded filament. Once it has reached the target temperature, purge the extruder by clicking on the arrow.
At the same time, manually push the filament slightly in until you can see the material is coming out of the nozzle.
Keep pressing the purge button until you have extruded approximately 5 cm of filament.
Then try to unload the filament using the automatic unload assistant.
If you still can't unload the filament, then follow the steps explained in this video to do it manually: