How to upgrade my iPhone to the IOS7?

So I just bought an iPhone 4 off of amazon and it's a really old version of it and it is SUPER slow. I go to settings then general and it doesn't say upgrade ANYWHERE It doesn't let me get on any apps until I get it so I really want and need it. Someone please help!

Update via itunes.
iOS 7.1 speeded up the 4 so it's not that slow anymore.

If it's slow with the old OS, wait until you put 7 on it. I got my kid a new phone just because it was too slow to do anything.

I don't think you can skip-upgrade it to ios7. If it's 6.0.1 for example, you may have to update to 6.0.2. Go to settings and then About to check what software version it's currently on.

Just update it via iTunes. Seriously, you could have Googled this in less than 5 seconds before asking this. Why is it so hard for some people?