Update

It is a bad idea to install any beta on any device that you need on a regular basis. By definition, a beta version is one that still has known bugs. Some of these bugs may render your phone useless. But if you are a gambler, here's what you need to know: How to install or uninstall iOS beta software - Apple Support

Note that you must register for the beta program and get the beta version from Apple. Any other source could endanger your privacy and allow unscrupulous people to steal your data, as versions gotten anywhere else may be hacked.

Once you install the beta you can no longer get help in this forum or from Apple support; you can only get it from the beta forums, and they are more interested in learning about any problems than they are about helping you get around them.

Update the iOS software on your iPhone, iPad, or iPod touch - https://support.apple.com/HT204204

Read here carefully and understand what you are getting into if you install the beta:

Apple Beta Software Program - FAQ

Hi

Go to settings down to general -about -software update.

Cheers

Brian 🇬-1F1E7;

What iPhone model do you have?

Settings->General->About->Model

What iOS version is it running?

Settings->General->About->Version

OK... 11.2.6 is the current version of iOS. iOS 11.3 has not been released yet.

Join the beta program. Note that the beta is intended for testing. It should NOT be used on a device you rely on for work or important communication.