Go to the Git Integration for Jira app Atlassian Marketplace page.
Buy or evaluate the app with a free 30 day trial.
Login to your Jira account, if required, to proceed installation of the app. The license key is automatically configured into the app configuration for free trial licenses. For purchased license keys, see Managing license key.
More on Git Integration for Jira installation
Installation via Atlassian Marketplace (this page)