Developer mode is easy to get to, you just have to remember to enable it
As has been the case since Android 4.2, the developer options have been hidden by default from your Android settings menu. This is a good thing, actually, as it'll keep curious minds from potentially breaking things. It also helps keep your phone more secure.
But there are plenty of reasons why you would want to enable developer options on your Galaxy S5. If you want to plug in to a computer and do any sort of command-line work, this is going to be your first stop. Or if you need some extra debugging options as a programmer. Or, maybe you are that sort of curious creature who just wants to see how things work. Nothing wrong with any of that.
But, of course, you're going to first have to turn developer mode on for your Galaxy S5. Here's how to do it.
Go to the settings menu on your GS5.
Scroll down to the "System section and tap "About Device."
Now scroll down to "Build number."
Tap the "Build number" section seven times. (You'll see a warning that you're about to unlock developer options.)
Now back out to the main settings menu, and you'll see "Developer options" listed.
From there, you can enable any number of Galaxy S5 developer options at one time. We usually start with USB debugging. If you ever want to turn off developer options wholesale, there's a toggle at the top of the section. This won't hide the options from prying eyes, however. But it'll keep you from accidentally enabling any of the options.
