my 6 plus will not update to iOS 13. What can I do to fix this?

Did you try updating while connected to Mac notebook/desktop ?

Update your iPhone, iPad, or iPod touch - Apple Support

With this in mind, the iOS 13 compatibility list for iPhones and the sole iPod is as follows:

• iPhone 6S and 6S Plus.
• iPhone SE.
• iPhone 7 and 7 Plus.
• iPhone 8 and 8 Plus.
• iPhone X.
• iPhone XS, XS Max and XR.
• iPhone 11, 11 Pro and 11 Pro Max.
• iPod Touch seventh generation.

Nothing, the 6 series doesn't have sufficient RAM or a strong enough CPU to run iOS 13.