Rebooting is enough to get rid of stages 2 and 3, if present. To get rid of stage 1, you have to factory reset, and preferably overwrite the firmware too, even if it's already up-to-date.

More helpful than TFA or the FBI's announcements was this blog post, already featured on HN at least once and linked by commenters probably multiple times:

https://blog.talosintelligence.com/2018/05/VPNFilter.html