The tickets export and import functionality resides at Tickets Settings > Export & Import.
Exporting files
Your exported file contains the following:
Tickets
Milestones
Ticket Comments
Ticket Activities
Ticket Associations
Custom Fields
Custom Field Values
Followers
Ticket Reports
Note: It doesn't include attachments.
Once you are in the Export & Import page, click Export your tickets and it generates a link that contains your file.
Note: The tickets file is a .bak backup file.
Importing files
To import your files in another space, go to the same path (Tickets Settings > Export & Import). Browse for your file in your local machine, and then select Import from Json (Downloaded from this site). Click Import Tickets.
Note: If you check the REMOVE existing tickets box, it removes all of the existing tickets in that space. If you don't want to do that, do not mark it and your imported tickets are added to your space with the existing ones.
Remember to invite all the users to your space if you want them to continue working with the tickets they are assigned to.
Also, keep in mind that the person who imports the tickets is set as the creator of all the tickets.
If you have any questions or if you need any further assistance on this, email us at support@assembla.com