To sync Stacks data with Dropbox service, first you need to download Dropbox from here https://www.dropbox.com/downloading

After you've have configured Dropbox folder on your device, assuming you currently have Stacks file saved on a local folder, you just need to move your current Workplace folder from the local folder to a Dropbox folder following the Moving a workspace guide.

After the files have been synced, to access the workspace in the other pc/mac you just need to install Stacks and follow the guide to add a new workspace, Adding a new workspace locating the Dropbox folder you have just synced. 

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