Software update will not install

There are no updates for an iPhone 6 beyond 12.4.7; that is the end of the line for that message.

Here are the conditions that must be met to update iOS:

• You must have a fast Wi-Fi connection; you cannot update over cellular data or a cellular hotspot
• The phone can not be in Low Power mode
• The battery state of charge must be above 50%
• You must have at least 3 GB of free storage
• You cannot be connected through VPN
• If you are not planning for it to update automatically you should turn this option off, as it will only update when the phone is plugged in, connected to Wi-Fi and locked
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

Look among the individual apps for a large file. It will be near the top, because the list is sorted by decreasing size. Look for a file between 1GB and 2GB.

If all else fails you can update it using your computer. iTunes for a PC or a Mac that does not have Mac OS Catalina, Filer for Catalina. See—>Update your iPhone, iPad, or iPod touch - Apple Support

Did you ever have a beta version on your phone?

Go to Settings/General and scroll to the bottom. Tap on Profiles if it exists, then tap on any beta profile and delete it. Then restart your phone.

You may also have a stray update download. Go to Settings/General/iPhone Storage, look for a large update download and delete it. Then restart your phone.

Restart your phone. Verify that you are connected to Wi-Fi and not cellular data, and not through a cellular hotspot.