Every time you create a section within a proposal, template or in the content library, Proposify creates the first version. Every time you reuse that section in another proposal, it will save another version of it.
To see the list of revisions, click on the "last edited by" that appears above the section while editing it:
This will take you to the Revision History. Previous versions of this page are stored here. Here you can click on past revisions to roll back and make edits to old versions. When you edit an old version, it will automatically duplicate that version and make a new one so you don't overwrite other proposals.
If you'd like to set an older version to be your default, just click the "Make Default" link when hovering over the version.