My iPhone 6 can’t upgrade its software ,can anyone help me please

Hi, your iPhone 6 can only update to iOS 12.5.3 as Kurtosis12 stated, you would need a iPhone 6s series or newer to benefit from further iOS updates. Also Apple announced that the iPhone 6s series will get the iOS 15 update.

An iPhone 6 can't be upgraded beyond 12.5.3, it doesn't have the hardware needed to run the latest iOS.