How can I update iOS on my iPhone 11?

Please see:

Update your iPhone or iPad - Apple Support

An iPhone 11 will support the current iS version, 17.X. If you have not updated in a while you may need to use the "connect to a computer" instructions linked at the top of that article.

What iPhone model you have ? In case it’s iPhone 6 those models can’t be update beyond iOS 12.5.7

What iPhone model you have ?

The OP asked:

"How can I update iOS on my iPhone 11?"

In addition, the OP's tagline tells us that he has an iPhone 11.

Safe to assume that he has an iPhone 11.