How can I update my iPhone 6 to iOS 13.5?

What about software update iOS 13.5? More information would help. Is your phone compatible?

iOS 13 and iPadOS are compatible with these devices - Apple Support.

Update your iOS device.        

Get help with over-the-air iOS updates          

Update – Can’t or can’t restore 

If you see an error when you update or restore your iPhone, iPad, or iPod          

muhammedismail wrote:

iPhone 6 software update iOS 13.5

[Re-Titled by Moderator]

You don't. The iPhone 6 is not compatible with iOS 13.