Phone is 4G but showing 3G

Yes, it is an LTE / 4G capable phone.

Your carrier needs to be providing 4G service in your area and you need a compatible SIM.

I don’t know where you’re at, but in most of the world, 3G networks no longer exist or are being phased out.

You cannot do it yourself. That is entirely up to your carrier.

If it is connecting to a 3G network at your current location, that means there is no LTE/4G service available there.