I want to install iOS 13 for iPhone 6?

You need 2GB+ RAM for iOS 13 and above. See the pic at the bottom of the post to understand what you have.

You need to correctly Identify your iPhone model - Apple Support

• iOS 14.7.1 —> 6S / 6S Plus, SE (1st Gen and 2nd Gen), 7 / 7 Plus, 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max, iPhone 12, 12 Mini, 12 Pro, 12 Pro max
• iOS 12.5.4 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, 

Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support

Unfortunately, your phone is hardware-limited to iOS 12.5.4.

You need a new phone. An iPhone 6 cannot update past iOS 12.5.4.