Depending on your country, documents names may not be relevant.
For example, an Estimate can be called Quote or Proposal depending on companies.
To change the name of a document, go to Settings > Sellsy Add-on > Sales Management > Documents layout and open (or create) a layout.
Then type the desired value in the Document name field:
Now, all new documents using this appearance will display the new title:
Note: you can associate this layout with a document type in the layouts management page.