IPhone 6s Plus not stable with the ios 15.6

Mohammed-issa wrote: "Why is it always restarting"

With an old iPhone model, the reason may be a weak battery...

Does your iPhone 6s Plus still have its original battery?

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

Settings > Battery > Battery Health

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Try this -> Restart your iPhone -> Restart your iPhone – Apple Support (UK)

See also -> About iOS performance on your iPhone, iPad, or iPod touch

About iOS or iPadOS performance on your iPhone, iPad, or iPod touch - Apple Support