Currently on btwb there is no personal "view" of metrics across workouts right now. Whichever units the workout used when originally created will be displayed. So if there's a workout you're looking at that's listed in kilograms, it was originally created with kilograms.
When you build and log your own workouts though, you can choose whatever units you use. When adding a weighted movement to your workout, toggle on the "weight" characteristic. Add a load, then click on the units to reveal a list of units options.
Select your units, then proceed through the final steps to logging a score for your workout. Results will be displayed using whichever units you selected when building the workout.
You won't be able to set a specific preference for the unit of measurements associated with your account, but if you consistently post weights in kilograms, for example, the weight input box when posting will start to automatically default to kilograms instead of pounds. You should start to see this happening after only a few posts.