Go to Settings, search for Network/ SIM/ Internet. You can also type Data or Data Saver in the Settings menu to quickly to the desired sub-menu.
Tap the Settings icon next to the service provider's name under Internet.
Scroll down and select Data warning & limit.
Tap on Mobile data usage cycle, and you will be able to set the duration of your data use cycle.
Tap on the Set data limit option to enter the data limit.