How to change the internet settings on my Iphone from Wi-Fi to 3g?

I use Wi-Fi on my Iphone when i'm at home but when i go out i want to use 3g but i don't know how to switch the settings…

Once you leave your Wi-Fi signal the phone should automatically switch to 3G, unless you go to another location with a saved Wi-Fi Connection

General-> Cellular-> and then switch on Enable 3G.

Go to settings -) general -) cellular data then activate 3g (if it doesn't activate the 3g it will probably go to edge, this means you are distant from your carriers tower.

EASIER WAY
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if your iPhone is Jailbreaked, then search for sbsettings in cydia and install it then activate it by swiping your statues bar
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hope this answered your question!

Turn off Wi-Fi and it will go to the higest which is 3G or LTE