How to upgrade my iPhone 4 to iOS 6?

I've been trying to figure out how to upgrade my iphone4 to iOS 6, and can't seem to do it

You have two methods for downloading and installing iOS 6. You can simply update your iOS device over the air via the device's Settings, or you can connect it to your computer and use iTunes.

Method 1: Over the air
The easier of the methods is to grab your iPhone, iPad, or iPod Touch and update wirelessly. Before doing so, however, it's wouldn't be a bad idea to sync your device with iTunes if you haven't done so in a while. There have been instances with past iOS updates where users have reported the new iOS didn't install correctly, bricking their phone and forcing them to restore their phone before trying again. So, if your iOS device has talked to iTunes recently, go ahead and go to Settings > General > Software Update.
You should see iOS 6 listed, with a Download and Install button below it. Tap the button, agree to Apple's terms and conditions, and either heed or ignore Apple's advice to connect your device to a power source during the download to save battery. After iOS 6 installs, your device will restart, which was a rather lengthy process in my experience (20 minutes or so, after the roughly 45 minutes it took to download and install). After restarting, you'll be greeted by a shiny, new iOS 6 lock screen. Slide to open your device and click through the following four setup screens:

Method 2: Update via iTunes
If you are near your computer with iTunes, it might be faster to update this way. In my experience, updating my iPad 2 via iTunes was faster than updating my iPhone 4S over the air. The first order of business is to update iTunes to version 10.7. Open iTunes and then click iTunes > Check for Updates to install the latest version.

Once iTunes is up-to-date, connect your iOS device. Click on your device under the Devices header in the left-hand column and click Check for Update.

A pop-up window will appear, alerting you that a new iOS version is available for your device. Click the Download and Update button. (Or, if you are going to need your phone in the near future, opt for the Download Only button to download it now and install later. IOS 6 is a 626MB download for the iPhone, which could take an hour or more, depending on your connection and the amount of traffic hitting Apple's servers.)

Click through two more setup screens that show you iOS 6's many new features and Apple's terms and conditions to start the download.

Once the download is complete, keep your iOS device connected until it restarts. After restarting, you will need to hop through four setup screens before you are off and running with iOS 6. On my iPad 2, the process looked like this

The current iOs is 7-plus - so are you asking about ROLLING BACK to 6? If you just have so much junk on your phone that it can't take the upgrade data, then you need to clean out things - maybe do a backup with your computer or the Cloud, delete a lot of things, run the iOs upgrade, and then see what you have room to replace. If you have a huge amount of pictures or music on your phone, then you need to prioritize - stick things in the Cloud, because NOBODY needs to walk around with every cute selfie they took, all their friends pictures, their entire music collection on their phone! I have been a professional musician full-time since 1973, and have VERY LITTLE music on my iPhone - I have a loaded mp3 player, and my computer has quite a bit - and there are several thousand CDs, vinyl albums, tapes, etc. All over the 2 studios here in the house - but I keep my phone clean and lean. Pictures go to the PhotoStream and directly to my computer (you need to set that up) - and I do not save a ton in Camera Roll, either.

Go to settings>general>software update. It will tell you if there's not enough room on the phone for the update, if this is the case then just uninstall free apps, you can reinstall later. You also have to be connected to Wi-Fi and preferably plugged in and charging

Actually, the iPhone 4 gets iOS 7.