From The Document Foundation Wiki
How to remove the Read only attribute?
The▸ ▸ menu allows you to force a file to open in read-only mode.
However when you open such a protected document, the▸ ▸ menu is not available.
To go back to unlock the document, click on the Edit File button on the Standard Toolbar.
The title bar still displays (Read Only) until the first modification is done in the document. The▸ ▸ is immediately available again.