Take a look to redmine.ociotec.com/projects/redmine-plugin-scrum or redmine.ociotec.com/projects/redmine-plugin-scrum/wiki for further details.
If you want to access to the plugin source code repository you need to support this plugin via www.patreon.com/ociotec (check tier with GIT access).
If not you can always download a tagged version of it at: redmine.ociotec.com/projects/redmine-plugin-scrum.
As any other Redmine plugin (you need GitLab access, take a look to www.patreon.com/ociotec in order to gain this access):
cd <redmine-path>/plugins git clone ssh://[email protected]:10022/redmine/scrum.git cd .. bundle exec rake redmine:plugins:migrate
Finally restart your Redmine installation.