How to update my iPhone 5s to iOS 14?

Unfortunately, you can not.

The iPhone 5S is not compatible with iOS 13 or higher.

In addition to not being updatable, your phone will no longer work as a phone in the near future, as carriers shut down their 3G networks. AT&T and Sprint have already completed the shutdown, T-Mobile has started, and will be completely shut down by July 1, Verizon by December 31.