Changing device types

If you want to target on desktop or mobile only, you can do it in the ad set settings. If we're talking about a new ad set, go to the settings and select Placement. Afterwards, you'll see the option to change the device type in one of the options below:

If it's an already existing ad set in a running campaign, you need to edit the ad set and only then will you come across the option shown.

Please note that if you're building an Instant Experience ad, you should select mobile only.