Most likely if dripping from hose connection when off you can replace seal. See videos below, course have shut water off.
Back flow preventer usually has the soft set screw broken off as described by other posters. So will have to find it (usually on bottom when make it hard to drill out)( which usually have to be careful not to ruin the few threads on hose bib. When I replace my backflow preventers I never break the set screw head off so I can easily replace it in few years.
https://www.youtube.com/watch?v=yqGx9R2bozk
Easily Repair a Leaky Spigot or Hose Bib | DIY Plumbing Repair - YouTube
Back flow preventer or just twist it off. But make sure hold the bib housing so it don’t rotate.
https://www.youtube.com/watch?v=liJSfA4yN1g
More how to utube video’s if don’t like these.
If don’t want fool with it recommend as others have suggested mike Scott plumbing.