how to update my iphone 6splus ?

That will help, but tap on instagram, for example, and there will be an option to offload app. Keep doing that for other apps until you have 1.7 GB free. Those apps will reload the first time you use them. And the update will free most of the space in Other.

Go to Settings/Software Update and tap on Update Now.

There are two obvious reasons:

And you need to connect to a WiFi network.

Read the screen shot you posted. It tells you what the problem is.

There is another problem it doesn’t mention; you also need a Wi-Fi connection to update.

Offload some apps: Settings/General/iPhone Storage. Tap on an app and choose Offload App. This will preserve app data but free up the space used by the app’s code.

Stedman’s answer will be the same as mine.