Please how do I switch my sim from 3G to 4G in my iPhone m

Firstly check if your sim is 4G enabled. In your message box sms 4G to 131. If it confirmed that your sim is 4G enabled. Follow these step
Select Settings.
Select Mobile Data.
Select Mobile Data Options.
Select Voice & Data.
To enable 3G, select 3G.
To enable 4G, select 4G.