Unable to download iOS 15.8.3 on iPhone 6s

What happens when you try?

Can you post a screenshot of the whole screen, it could reveal why you aren't able to update.

Make sure you:

• Are using a WiFi connection (not a hotspot).

• You have an active cell signal (You need both WiFi and cellular service to perform the upgrade).

• Are not using a VPN connection.

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 10 gigs of storage available.

Click on this link on what to do if you can't update your phone -

If your iPhone or iPad won't update – Apple Support (UK)

You should be able to update an iPhone 6s to the latest release of iOS 15 – currently iOS 15.8.3.

Note that if, by any chance, you actually have an iPhone 6, you cannot update it past the latest release of iOS 12. Despite having a very similar name, the iPhone 6 (no "s") is an older model,

Yea I can’t make space on my iphone 6s

I would suggest a factory reset of the device becuase then you will surely can update your phone!