Tickets: Configuration > Tickets > General Settings > *scroll to the bottom* > Import Tickets
Users: Configuration > Users > General Settings > *scroll to the bottom* > Import Users
All imports other than Tickets, Actions, Client & Supplier Contracts and Knowledge Base Articles can also be used to update existing information - you just need to set a Unique Identifier in the Import set-up to ensure the existing records are matched. Please see the example below for a Clients Import:
The unique identifier here is Name, so if the name matches correctly between the spreadsheet and Halo, then that record will be updated, rather than a new record created.