Jeff Thomas's answer is correct (a.k.a. "the bottom of every page"). If you have admin privileges on your Bitbucket Server instance you can also get detailed version information in the Administration area:
Click on "Administration" (top right corner, a little gear icon).
And then in the left menu, at the very bottom, under "Support", click on "Support Tools."
And then click on the "System Information" tab. You should see something like this:
Base URL: http://vm.bit-booster.com/Bitbucket Build Number: 4007001 Build Timestamp: Tue Jun 14 04:28:14 EDT 2016 Build Version: 4.7.1
Hey Community! We’re willing to wager that quite a few of you not only use Bitbucket, but administer it too. Our team is excited to share that we’ll be releasing improvements throughout this month of...
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