They make battery holders for the 3.6 volt cylinder batteries, also known as a 1/2 AA style battery. If you wanted to you can solder one of those battery holders to the board, then easily install a 3.6 volt battery. They make the 3.6 volt batteries with and without solder leads. These 3.6 volt lithium batteries last a pretty good while, so you may be happy just replacing the dead battery by soldering in a new one with leads and not even use a battery holder. Be sure to watch where the + end of the battery connects.
The type of connectors on the edge of your mpu board (circled in your photo below) make me think this is an IGT S machine, not an S+. But the experts here will say for sure.