how to convert 4g into 3g

[AC Question]

I have samsung alpha so tell me how to convert 4g into 3g

 

[Tim1954]

You can't "convert" them.
If you mean how to set your device to use one or the other, you can do that in settings... Connections... Mobile networks... Network mode..
Mostly they are on auto and use whatever is around, so you don't need to be fiddling with that...