How to install iOS 14 in iPhone 5s

You can't.

iPhone 4S has a 32bit chip with 512MB RAM.

iOS 13 and higher requires 2GB RAM or more and a 64bit platform. iPhone 6/6+/5s has 1GB RAM

These are the iOS options based on iPhone Model --> You need to Identify your iPhone model - Apple Support

• iOS 15.2 —> 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, iPhone 13, 13 Mini, 13 Pro, 13 Pro Max
• iOS 12.5.5 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, 

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

There is no method to install iOS 14 on an iPhone 5s. The last version of iOS that it can support is 12.5.4

How can I upgrade my iPhone 6 to iOS 14

You cannot on iPhone 6, only to 12.