SAML SSO add-on licenses are managed by Atlassian through the my.atlassian.com platform. To upgrade your license please follow the steps below:

1. Login to https://my.atlassian.com with your Atlassian Account, ideally the one who purchased the license.
2. In the section Licenses find the license for the SAML Single Sign On Plugin. 
3. Click on the arrow to expand the view to see the license details.
4. On the right side, below the License key, find Actions. 
5. Select the Upgrade Link.
6. On the following Page of the Marketplace, under Upgrade User Tier, select an user size and a maintenance period.
7. Follow the rest of the Instructions to complete the transaction.