Apple Watch doesn't include a multitasking feature like its iPhone companion, so there's no way to show active apps or swipe up to force-close one. Watch is an extension of the iPhone, not a replacement, so including a fully-functional multitasking feature seems impractical.
So while you can't truly multitask, there is a simple trick that'll allow you to quickly switch to your last used application.
While using your Apple Watch, all you have to do is double-click the Digital Crown and you'll be directed to the last app you were using. Essentially, you'll be able to switch between the last two apps you were in pretty seamlessly with this feature.
While it may not be multitasking as we've come to know it, it is useful and very easily executed. I don't plan on switching between too many apps on the small display, so this action will certainly suffice.