I am assuming these are Mafac Centtre Pull brakes not Cantilevers on frame bosses ?
1) Best if you remove the appropriate wheel. This will give you room to pinch the brake blocks together which will raise the brake arms and enable you to remove a straddle cable end from its hook on the end of one of the arms. Usually only one end of the straddle cable is removable.
2) Slide the straddle cable out of its pull hook and all tension is now removed from the braking system.
3) Returning to the brake lever there should now be enough flex in the cable to push it to one side with a screwdriver. You may be able to manoeuvre the nipple on the end of the cable down and out of the brake lever which will give you even more flexibility.
4) Place a box or tube spanner over the nut and a couple of turns should be enough to make the lever a bit loose on the bars so you can slide them into the position you want.
5) When all ok tighten up the nut, pull the cable back into position in the lever housing, place the straddle cable back in its pull hook, pinch the brake blocks together and reinsert the straddle cable end back in the end of its brake arm. As you release the blocks tension will return to the braking system.
6) Refit the wheel and check block alignment to the rim and use the screw adjuster ( I assume you have one on each brake system) to tension the cable to the right feel for you at the lever.
Hope this helps