Depending on who put it together, it may either slide right off, or be "glued" to the tube with some silicone rubber caulk . Also look for any signs of zip ties or something that may have been put on by a 3rd party to keep it from falling off at some point.
You will probably need to disconnect the black ground wire the attaches to the metal strip or spring that touches the outside of the tube, and remember where to reattach it. The rest of the wires you can leave in place as they should allow you to move it out farther.
Be very carefull removing it. It should slide DIRECTLY off the back. Do not twist or rock it unless absolutely necessary, and if so, only move it no more than a milimeter each way to help unseat it, otherwise you will end up either breaking the pins out of the tube or cracking the neck as it's very very thin glass back there.
If there is caulk holding it on then take a brand new sharp utility knife and cut a line through it in a circle all around the tube until you reach the glass of the tube, pressing VERY VERY lightly (thats why you need to use a brand new blade). Lightly pull on the neckboard while cutting to see where you may need to cut deeper.
Good luck