With the introduction of on-screen navigation buttons in Android, it was only going to be so long before some of the manufacturers begun to give users options on customizing them. This has long been possible for those who had the courage and the know-how to root their devices and flash custom ROMs, but just in the last year or two companies like LG and Samsung have started to incorporate customizable navigation keys into their flavor of the Android UI. HTC has joined this group of OEMs giving users choice and has added this function to the HTC One M9 with the inclusion of the Sense 7 software, and should you be interested in toying with it, you won't have to work too hard to do so.
HTC has been generous here and has implemented the ability to swap around the existing nav keys to meet your personal tastes for the order that you wish, but they have also allowed you to add in new nav keys which have different functions. For instance, you can add in a new navigation key which lets you tap to open up the notification panel. Tapping this again lets you transition from the notification panel to the quick settings panel. There are more navigation buttons as well which let you power off the display, toggle auto-rotate, hide the navigation bar at your leisure and there is even a button which strictly opens up the quick settings panel so you don't have to double tap the notification button.
Accessing the interchangeable navigation keys can be done by first going into settings and getting to the menu for personalization. This is where you'll find the navigation bar options as well as any other customization factors. Enter in to change navigation buttons and simply tap to add the ones you want in addition to the existing buttons for recents, home and back. Alternatively, you can also remove all three standard navigation buttons and replace them with any combination of the other ones HTC has put in. It's worth noting that you can only have four selected at any given time. Once you have what you want, simply hit "done" to save your selections and that's it. Change at will whenever you want something new.