There are 2 ways of installing depending on whether your JIRA server has a direct connection to the Internet

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Log in as a Jira administrator to carry out the following instructions.
Ensure that the plugin version to be installed is compatible with your Jira version. (View: Compatibility matrix)

Install Plugin

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Server With Internet Access

Under the Plugins section, click on Find New Add-ons
Enter Calendar HeatMap Gadget in the texbox labelled with Search the Marketplace
Click on the Install button to install the plugin
Wait for the installation process to complete and click on the Close button
Click on the Manage button to proceed with the configuration

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Server Without Internet

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Access

Upload .jar files, downloaded from Atlassian Marketplace

Under the Plugins section, click on Manage add-ons
Click on the Upload add-on link
Click on Choose File button and select the latest version of the plugin from the Marketplace (e.g. calendar-heatmap-1.1.0.jar)
Click on Upload button to upload the plugin
A confirmation dialog box will appear

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Generate/Purchase License Key

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A free license is required for the plugin to work.

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admin to request a license
Go to the Universal Plugin Manager
Click on the Calendar Heatmap Gadget under the User-installed Add-ons section to expand the panel
Click on the Configure button
Enter the details inside the following screen
Click on Submit button to save the details