How can I backup my iPhone now?

Start here on how to do a backup: How to back up your iPhone, iPad, and iPod touch - Apple Support

what do you mean that your phone is behaving strangely?

tamandani64 wrote: "My iPhone 6s keeps on behaving strangely"

An old model iPhone such as your 6s may be "behaving strangely" due to an aging original battery or a nongenuine replacement battery. So...

Does your iPhone 6s still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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