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Just change to 8MP on manual mode and you can then edit what you need.
The HDR works only on certain conditions, one of them is that the resolution has to be 8mp or below, the image stabilizer has to be OFF and the main settings need to be "Manual" as "Superior Auto" or other apps don't support HDR, however as I love HDR photos when outdoor, I recommend "Google Camera" where you can use the HDR also at 20mp.