This is applicable to the Ultimaker 2, Ultimaker 2 Extended, and Ultimaker 2 Go.
When the feeder is unscrewed, it basically takes itself apart. However, the screws that hold the feeder assembly together also hold the motor on. To keep the motor from falling, you'll need to hold the motor while you take the feeder off. You can access the motor by removing the left cable cover. There are two screws that hold the cable cover, one on the back panel and one on the side panel (older machine will only have one screw). You can find a diagram of the screws here. The Ultimaker 2 Go cable covers pop into and out of place.
Unfortunately, it doesn't put itself back together. The instructions below will help you put the feeder together again. You can find the names of the feeder parts here to help you.
Here's how to get that feeder back in working order:
1. Place the half of the black plastic case with the post in front of you. Put the white lever onto the post. The post goes through the hole in the corner of the lever.
2. Once the screw adjusting the tension has been moved, it has to be reset so to speak before the case can be put together again. Place the white holder in place (long arm to the left) without the spring. Adjust the screw so that it sits in the cutout without pressure.
3. Now that the screw is the right height, take the holder back out. Place the spring against the bottom of the holder. Place the other end of the spring on the straight arm of the lever. Compress the spring until you can get the holder back into place.
The spring should be straight. A crooked spring impedes movement of the filament. If your spring is crooked (see below), push the spring into the proper place with your finger or take it out and try again.
4. Set the white collet in the hollow at the top of the case. Set the silver ring in the hollow at the bottom of the case. Then close the case.
5. When the extruder goes back on the machine (and you're looking at the back of the printer), the white indicator should be on the left side for an Ultimaker 2 Go and the right side for an Ultimaker 2.
Hold the motor inside the printer so the knurled wheel protrudes through to the back. The wires should exit the motor at the bottom. Do not pinch any wires between the motor and the back panel. Put the case in place. Screw the case onto the motor. Recommended tension for the 4 screws is hand tight, and then slacked off by 1/4 turn.
6. Replace the Bowden tube. Secure the blue clip under the white collet. Adjust the tension screw so the indicator is on the first hashmark. Replace the corner cover using the 2 8mm screws you removed earlier.