Back up your internal storage (connect to pc, crtl-c ctrl-v)
You can backup your app data with helium
I dont use one and i dont know many who do use them, if you have much sensile data, u might want to get one.
Imo a anti virus app is only useful if you are rooted, so the antivirus app has more rights than usual apps. If u r unrooted and the antivirus has same rights as maleware apps its kinda useless
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