The answer is either. The RH arm is the one that is tightened during adjustment. If there has been a little movement during use you may be lucky. If not,it is not uncommon to have to heat them to remove the arm. Undo the tab and gice the arm a light hammering around as far as you can. Apply a spray into the splines to assist in the freeing. Hopefully this will work.
Before fitting the new arm;
Release the tab on the LHS and slacken the bolt. Ensure that the LH arm will slide off at least 1/4". Fit the new arm to the RHS and tighten the bolt and lock the tab. Refit the lift rod. Adjust the LH screw so that the linkage just falls under its own weight. Lock the tab.
DavidP, South Wales