Hello and welcome to the forum.
The lower arms are a b4stard to remove and the joints are even worse to replace!.The arm is held by a long pin that runs the length of the arm,you need to remove the large nut from the front of the bar (30 or 32mm from memory) and then you can undo the bolts at the very back of the arm holding on a box type unit.Once this removed,you can then pull the pin out and the arm will come loose.
I changed a couple of these the other day and the joint was bl00dy tight to remove and press in,it would probably be an idea to remove the arm yourself and then take it to a garage or engineering workshop to have the old joint removed and the new one fitted.
Any problems with getting the arm off and I'll take some pictures of the ones we have at work and try and explain it a little better.
EDIT;Oh,and just to cheer you up,the upper arms are even worse!.