how can i update ios 15 in iphone 6 s

1.

You can update the iPhone 6S to iOS 15, see this Apple article: iPhone models compatible with iOS 15

https://support.apple.com/en-gb/guide/iphone/iphe3fa5df43/ios

At the top left you can select the correct iOS.

Go to: Settings -> General -> Software Update

2.

See this Apple articles below

Clear Other storage on your iPhone or iPad

https://support.apple.com/en-us/HT211686

How to check the storage on your iPhone, iPad and iPod touch

https://support.apple.com/en-gb/HT201656

If you need more space for an update on your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT203097

What's the difference between device storage and iCloud storage?

https://support.apple.com/en-us/HT206504