JIRA Server
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.
JIRA Cloud
Agile Cards Cloud Plugin can be installed directly from UPM in your JIRA Cloud instance ('Cog' Menu > Add-ons > Search the Marketplace field) or from plugin page on Atlassian Marketplace. You can choose free trial for one month to check if our plugin suits your needs. More detailed documentation about installing JIRA Cloud plugins can be found here: https://confluence.atlassian.com/display/AOD/Subscribing+to+JIRA+Add-ons. Please remember that, contrary to the JIRA Server plugins, in JIRA Cloud all plugins are sold in a form of monthly subscription.