From the Campaigns tab in either Google or Bing Campaign Manager, scroll down to the campaign table and locate the dropdown for Segment:
By clicking on this drop down you can select Device which breaks down your campaign details by Mobile, Tablet, and Desktop performance.
For your desired device, click on the blue percent link to the right of the device name to open the Bid Modifier menu:
Apply your desired values and click Set to finish and save. This will immediately update your settings in either Google Ads or Microsoft Advertising, so no additional steps are required.