I can’t update my phone iPhone 6splus

That’s iOS 15.1 showing, and IOS 15.2 is current, so you’ll want to delete any partial download of 15.1, and download and install 15.2. (Settings > General > iPhone Storage, and look through the list there for any remnants of an iOS 15.1 download and delete that.)

Above reply’s links are good, you’ll need Wi-Fi, and what you will want to follow.

See if these links help...

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

Updating iOS: common issues and solutions - Apple Community

You are not connected to wi-fi. You need to be connected to a stable wi-fi network, the phone needs to be at least 50% charged, and it should be connected to power before you update.

In addition to the advice above, note that iOS 15.1 is no longer a valid update. You will need to delete the existing downloaded file and attempt the update again.