How can I update iPhone 6 to iOS 13 ?

You can’t. Your 10 year old hardware cannot run any version higher than 12.5.7. If you need a later iOS version you will have to get a newer iPhone.

Choose a version, and see what model phone you will need:

• iPhone XR, XS, SE 2nd or 3rd edition, iPhone 11 and newer: You can update to iOS 17.4.1
• iPhone 8, and iPhone X you can update to 16.7.7
• Any iPhone series 6S and 7, and original SE: You can update to 15.8.2
• iPhone 5S or 6: You cannot update to iOS 13 or higher; you can update only to iOS 12.5.7
• iPhone 5: You can update only to 10.3.4
• iPhone 5C: You can update only to 10.3.3
• iPhone 4S: You can update only to 9.3.6
• iPhone 4: You can update only to 7.1.2