How can I change the resolution in Lumia 830?

Based on the reference indicated below, you should be able to shoot in DNG format if you have Denim installed on your Lumia 830.

As for changing the resolution, not sure if you can do that. You definitely can't go higher than 10 MP, but you should be able to go lower. Have you tried changing the aspect ratio in your camera settings, as well as using a different camera app?

Designing PureView for Lumia 830 - Conversations: The Microsoft Devices blog
 
I thought Lumia Camera 5 had DNG capture options.

As for other resolutions, you probably want to consider another photography app like ProShot.
 
Cam 5 in the 830 has a max of 9MP (4:3) not 10MP

Settings
photo
Aspect ration 4:3
Main camera capture mode
will be 9MP not 10MP
 

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