How to back out of an app in new HTC phones?

I have bought new HTC phone. I’m using supersonic phone. I have read in a thread here to back out of an app, how do I do that? I am new to Smartphone’s. Also, to save battery life, I think I read to turn off data. I have already turned off 4G. If I turn data back on when checking FB, etc, does it give me the most updated posts? Any hints would be greatly appreciated.

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