Once you have an image built, you can flash it onto your EFR device (either a EFR32 development board or the Thunderboard Sense 2) over USB connected to your development machine. This can be done using either Simplicity Studio or the standalone Simplicity Commander.
A link to download Simplicity Commander's standalone version is located along with documentation in the
Simplicity Studio is a complete development environment and tool suite. It has the ability to discover USB connected development boards and flash them.