How can I reduce a data usage on my Nokia Lumia phone?

if your network service provider does not charge you a fixed fee for data transfer, there are a few things you can do to reduce your data usage .

Set mail download to manual if your phone syncs mail frequently, it uses more data. You can choose how often your phone automatically syncs mail.

• Tap the mail account whose settings you want to change.
• Tap the 3 dots (...) at the bottom right of the screen.
  
• Select Settings > Sync settings.
• Tap Download new content, and select a time. If you don't want your phone to sync mail automatically, select manually.

Stop data roaming: Connecting to the internet when roaming, especially when abroad, can raise data costs substantially. Data roaming means using your phone to receive data over networks that your network service provider does not own or operate. To make sure you are not using your phone to roam, on the start screen, swipe left, tap Settings > Mobile network (cellular), and switch Data roaming options to don’t roam.

Switch your mobile data connection off: If you want to make sure you’re not using data at all, you can switch your data connection off altogether. Tap Settings > Mobile network (cellular), and switch Data connection to Off.