We are having trouble getting the old cartridge out of the holder to replace it with a new one. Is there a trick to it?

The Brother MFC-240C uses an individual cartridge for each color (black, yellow, cyan, and magenta). When one of the cartridges is empty or near empty, the printer will tell you to replace that ink cartridge.

You can buy the cartridges individually, or you can buy a pack of all three color cartridges or a pack of two black cartridges. To replace one of the ink cartridges, follow the steps below.
All four of the cartridges are located on the right front side of the printer, behind a gray plastic cover. Pull the cover forward from the side, where there is a slight cutout for your finger to grab. When the cover falls forward, you'll see the four ink cartridge holders, coded by color.

For the ink cartridge that you need to replace, push down on the purple handle for that color. The ink cartridge holder will open and fall forward, revealing the ink cartridge itself. Push down on the cartridge holder and the cartridge should pop out toward you a little bit. Grasp the cartridge and remove it.
Open the new cartridge and remove it from its wrapper. Discard the yellow cover, unless you plan to use it for the old cartridge.

Insert the new cartridge into the correct slot (from left to right, they go black, yellow, cyan, magenta) so that the word "brother" is right-side up and located toward the top of the cartridge. Once the cartridge is most of the way in, push the ink cartridge holder up by the purple handle until it snaps into place.

If you are replacing more than one color, repeat this procedure for each cartridge. Then, when you are finished, close the ink cartridge cover.

If you were out of ink or low on ink, the machine should recognize the new cartridge and perform the calibration process. If it doesn't automatically recognize the new cartridge, it will ask if you have changed one or more of the cartridges. Press "1" for Yes when it asks about a color that you replaced, or "2" for No if it asks about a color you didn't replace.

Once the printer comes back up to the ready state, print a color test page to verify that everything is working. If the test page comes out okay, the new cartridge is fine and you can either discard the old cartridge in the trash or save it to recycle. (But don't get rid of it before you are sure that the new cartridge is recognized and working correctly.)

The Brother MFC-240C uses an individual cartridge for each color (black, yellow, cyan, and magenta). When one of the colors is out of ink, the printer will tell you to replace the ink cartridge. To replace a cartridge, follow the steps below.

All four cartridges are located on the right front side of the printer, behind a cover. Pull the cover forward from the side, where there is a slight cutout for you to grab. When the cover falls forward, you'll see the ink cartridge holders.

For whichever ink cartridge you are replacing, push down on the purple handle for that color. The ink cartridge holder will open and fall forward, revealing the ink cartridge itself. Push down on the cartridge holder and the cartridge should pop toward you a little bit. Grasp the cartridge and remove it.

Open the new cartridge and remove it from its wrapper. Discard the yellow cover, unless you plan to use it for the old cartridge.

Insert the new cartridge into the correct slot (from left to right: black, yellow, cyan, magenta) so that the word "brother" is right-side up and located toward the top of the cartridge. Once it is most of the way in, push the ink cartridge holder up by the purple handle until it snaps into place. Then close the ink cartridge cover.

If you were out of ink, the machine should recognize the new cartridge and perform its calibration. If it doesn't automatically recognize it, it will ask if you have changed one or more of the cartridges. Press "1" for Yes.

Once you have printed a test page with all four colors to verify that everything is working, either discard the old cartridge in the trash or save it to recycle. But don't get rid of it until you are sure the new cartridge is recognized and working.