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Agile Cards plugin is hosted at Atlassian Marketplace, so the easiest way to install the plugin is to go to Administration -> Add-ons -> Find new Add-ons page in your JIRA, search for Agile Cards and click Buy Now or Free Trial button. You can also download the plugin directly from Atlassian Marketplace and upload it to your JIRA via Upload add-on link on Administration -> Add-ons -> Manage Add-ons JIRA page.
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