If you don't have any unsynchronized notes, and you do not have Local notebooks, please skip to "Move your existing Evernote database." If you do have unsynchronized notes or local notebooks, follow these steps to export that content:
Select all unsynced notes, or each local notebook, then right-click and select Export Notes…
Choose 'Export as a file in ENEX format (.enex)', then click Export
Detailed instructions here
Move your existing Evernote database:
Open Evernote for Windows
Select File > Sign out… from the menu bar
Select Evernote > O