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Navigate to the asset and select ...More > Check-out/Lock.
Upon locking an asset, you can preview the working copy. It's shown first by default, and you can switch to the Current Version using the dropdown next to the title.
Depending on your review, choose to Commit Changes, Break Lock, or Reassign Lock.
All your working copies can be viewed under the 'Locked Assets' section of the 'My Content' menu.
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