There are lots of ways to do it. Simplest is to remove the two wires on the back of the amp meter and join them. Clean thoroughly, bolt together and tape or cut, strip, solder and shrink wrap.

Then add a voltmeter to monitor the system. Connect the voltmeter to a key switched source - like the key switched accessory terminal on the fuse block - and you've got it.

Another option is to convert the ammeter to a volt meter. A guage house can do it or it can be done DIY. If you make this change make sure the voltmeter is fed by a switched source. If not, it will slowly drain the battery.