I would like to change the default photo viewing app from Gallery to A+ Gallery on my Galaxy S5 running Marshmallow 6.0.1. I have looked at several instructions on how to do this, but all have steps that involve buttons that my phone does not have. Like no cog in the upper right hand corner on the application screen. And when I go to default applications, photo viewer is not one of the options. Any help would be appreciated.
Well there really isn't a default viewer on a device for looking at images. You can use whatever gallery you want. You just have to install said app, then add the icon for this app to the screen you want it on. Then, when you want to view an image, you tap that app icon. You can remove the icon for the other gallery from your screen so that it isn't accessed by accident.
Now other apps, the camera, etc., are a different story. They may use a default one for their access. In such a case you would have to check that apps settings to see if there is a way to change what app it uses to view with. There is no guaranty that it will have the option to change it though. Every app is different.