Android - Reset the request permission dialog on Marshmallow
You need to manually allow the permission on that specific app to reset the request permission dialog.
- Open the App info (either by dragging from app launcher, or Settings - Apps - [app name])
- Select Permissions
- Enable the permission you denied with "never ask again"
- (Optional) Disable it again from here; note that by this time, your app will request the permission again when needed.
You can also uninstall and reinstall.
The accepted answer is of course correct, but as an Android developer, you probably know how to adb uninstall <package> / adb install <apk> faster than navigating through settings.