On your Windows 10 PC, open the Notifications section by clicking on the text message icon at the bottom right of the screen (it could also be empty if you have no notifications).
Click on Connect. If you don't see Connect click expand above the four boxes at the bottom of the section.
After clicking Connect, you should see your Firestick listed if both devices are on the same Wi-Fi network.
Click on the Firestick listed and it should connect. You should now see your PC screen mirrored on the TV that the Firestick is plugged into.
Duplicate — This is the default projection mode and basically mirrors everything that’s happening on your PC. This is ideal if you want to share something on a larger screen but won’t need to use the PC. By default, Windows 10 also silences any notifications so they don’t pop up while your sharing something.
Extended — This enables using the Firestick and your TV as a second screen. It’s the better option if you’re looking to stream from a PC to a Firestick while using the former.
Second screen only — This mode lets you use the larger screen as the primary display.