please help me to update my iPhone

For recent iOS versions, you will want a newer (used, refurb) or new iPhone, as iPhone 6 Plus is supported on iOS 12 and no later. There are no upgrades, hardware or software.

iPhone 6s or iPhone 7 can get to iOS 15.

iPhone 8 and later can get to iOS 16.

iPhone Xs is the minimum for the iOS 17 preview.

An iPhone 5s or iPhone 6 isn't capable of 13 or above; they require at least 2GB of RAM. If you're trying to install 12.5.7, describe what happens in this thread.

(240605)

Go to settings then click general then click software update

iOS 12.5.7 is the most recent update for the iPhone 6 Plus. You probably already have that, correct?

If yes, there are no more updates for the iPhone 6 Plus.