I can’t download ios15 on my iPhone 6s

Do you have a 6 or 6S? Go to Settings > General > About and look at the model number.

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.5

• If you have an iPhone 6S through 11, you can update to 14.8.1 or 15.1

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch -

https://support.apple.com/en-us/HT204204

If you do have a 6S and you still can't update -

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).

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

• Are not using a VPN connection.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 6 gigs of storage available.

You can also check out this article for help -

If your iPhone, iPad or iPod touch won’t update

https://support.apple.com/en-gb/HT201435