Removing high data usage icon
If you see the icon High data usage warning,you must have accidentally checked the box for setting a data limit.
There is a warning at 2GB and a limit at 5GB if you so choose. Many providers have unlimited data plans.
You'll want to go to:
Settings -> More (Under Mobile Network) -> Usage and then uncheck Set Data Usage Limit.
(If it is greyed out, toggle your mobile network on and then it will let you select it.)