Yes, remove the dodgy cell and put the working one in. Make sure is secured and it doesnt matter if you use soldering gun or iron as long is powerfull enough to heat up elements fast enough so you dont heat up the cells. The tabs (or whatever you call them) are heat pressed together and sometimes get loose, they are made of alloy so it should be quite easy to solder. I should have made a tutorial but i never had time to do it