There are various ways to unfreeze the most recent version of a document in the current branch.
Right-click a frozen document, (or a folder or the root node) and select the Unfreeze command.
Use the Unfreeze tool on the property sheet of a:
document (Versions tab)
branch (Members tab – multiselection available)
configuration (Members tab – multiselection available)
You must have the Write or Full permission to unfreeze a document version. You cannot unfreeze any but the most recent version.