Follow these steps:
- Head to your Jira account
- Choose settings from the top bar
- Scroll down to 'Manage Apps'
- Search for 'Mobile for Jira'
- Click on 'Buy Now'
- A confirmation message will appear
- Click on 'Accept & Install'
- The app will automatically start installing
- A pop-up will appear asking for a license
- Click on 'Get License'
And you'll be done with the installation process.