What tool do you use to replace a 1.2V batteryA soldering iron doesn't work well?

A soldering iron should work if it is able to get hot enough and you have the right kind of solder. You need a solder that is made for electetonics. If you have an acid core or one used for Plumbing they will not work. The iron will need to produce enough heat to get the item soldered hot enough for the solder to flow and bond.