Jira doesn't really log this at all. Until version 6.2, where it is an option you can enable. There's a plugin for older versions as well (4.x upwards). However, both methods require that the logging be enabled BEFORE you make changes, as they work by recording admin actions. There's no way to go back and find out as it's not logged.
You might be able to get something from access logs though - if you've got anything in front of Jira to do it.
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