Just open the desktop app Preferences (from the bottom left gear button) and click on the Support tab.
In the Library Export section, choose either the ReadCube Archive or .bib export format.
To migrate your library - for example, to a new computer - choose the ReadCube Archive option. Select the "Export Files" checkbox to include PDFs. This option will create copies of the files in a new folder at the export location and may take a significant amount of time depending on your library size. If unchecked, only reference metadata will be exported.
To import your library on a different computer, simply use the regular Import screen to drag in just the .rc file. If the "Export Files" option was selected, we'll attach the PDFs automatically; just make sure the .data folder containing the PDFs is in the same location as the .rc file. All imported PDFs will be copied to the default folder location ("ReadCube Media" in your Documents folder). This setting can be changed in Preferences - General section.
For inter-compatibility with other software, please choose the .bib export option.
To export single or multiple selected references, instead of your entire library database, please see this related article.