How can I change IOS 12.5.5 to IOS 13.4 to my Iphone 6.

How can I change IOS 12.5.5 to IOS 13.4 on my Iphone? When I try software update , there is written that it is unable and try again. I have good internet connection. I don’t know what to do. I saw lot of tutorials on Youtube, but nothing works. Please, if anyone knows the way how to update this software , wtire to me. Thanks.