How can I use these peaches in a box cake mix?
I bet you can puree some peaches and their juice to use in place of the butter and water, respectively. Although, the texture might be better if you only replace half of the butter.  I wouldn't recommend replacing the eggs with peaches because that would make the cake quite crumbly.  Note that the peaches can make the cake a lot sweeter, especially if they're canned in heavy syrup.

Perhaps a better use would be to turn the peaches into an ice cream or sorbet to serve with the cake.  You could also add a small amount of peach to some butter and powdered sugar to make a buttercream frosting for the cake.