For more detailed information of how import the backup check Atlassian Documentation:
https://support.atlassian.com/jira-cloud-administration/docs/import-issues/The first thing we have to do is to recover the backup file we want to restore. Either by downloading it using the addon or a file that we have stored locally or in the cloud:
Once we have the file, we must navigate to the page intended to restore the backups of our Atlassian site:
In the top-right menu → [Jira Settings] Settings → Import Jira Cloud → Import Data