How to remove the back cover of the Swiss Army Victornoix pocket watch so to replace the battery?

The back will either have a small slot on one edge to pry it off or indentations around the perimeter for a case wrench to unscrew it. Specialized tools are used to avoid scratching the case. You best bet is to take it to a watch shop and let them do it. Most places charge between $3 - $8 to properly do it without damaging the case and they should also change the gasket to maintain water resistance if so equipped. Once you have it open you risk damaging the movement as you change the battery. It's not that it's a tricky procedure but you can easily damage something that will cost a LOT more than $8 to fix if you don't have experience.