First to remove the material, pull on the filament below the feeder while it is removing material. You might need to remove the bowden tube if the filament is stuck in the printhead(happened to me)
To clean the nozzle, follow these instructions:How to clean out the nozzle of an Ultimaker 2 3D Printer
(1) First buy yourself some of these great little strong syringes !
(or amazon.com, but eBay is cheaper. https://www.amazon.com/gp/product/B0026MER5Y/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1)
(3) Using the Maintenance->Advanced->Heat Nozzle menu on the printer, heat the nozzle to 250 degrees
(4) When the hotend is heated fully. take the syringe with your fingers and gently slide the pin up the centre whole of the nozzle... You should feel a little resistance from the blockage.. Gently push up with the syringe and then move the syringe up and down. Repeat this process and remember to take out the pin of the syringe after every time.. so the loosened material can escape the nozzle.
This whole process should take no longer than 1 minute !
(5) Then in the control panel... choose change material and extrude your new filament... wait until the filament really flows nice and smooth through the nozzle... when you notice that the filament is running well... end the material change mode..
You can also try the atomic method(just search it up on ultimaker.com), but that didn't work for me.