This is my (ex) Marathon Navigator battery change. It is pretty simple process.
The watch has a snap back back case design. The slot to open the case back is also pretty large. I suspect you can use a coin to pop out the case back. Well, since the watch was design to be used in the field, where there are no special tools, the watch has to be design to be as practical as possible.
Once you pop open the case back, you have full access to the battery. It is held down by the 'V' shape battery holder. The easier method is to hold down the metal plate and loosen the screw that is holding the plate down. DO NOT remove the screw. Once you have loosen the screw, just slide the metal plate away from the battery.
The Swiss ISA 1198 movement.
The stainless steel case back. Do not forget to lubricate the rubber gasket.
I took the hard way and unscrewed the screw. Well it was not difficult to put it back. :)
The battery, type 395 or 399 can be used.
The screw that hold the 'V' metal plate. Note the 'cut-off' at the 2 o'clock position (of the case back). This is the cut-off to facilitate the case back removal.
The new battery installed. The case back will just snap back in. No need for any special tools.
Adjust the time. If you notice the plastic on the crystal, yes it is new unused. And there you have, how to change the battery on the Marathon Navigator.