Hi @jaimeibar,
You only need to request the Migrators (via Get Migration Packages for Latest CE Release - OpenNebula – Open Source Cloud & Edge Computing Platform) if you want to upgrade to the latest version (6.4 now), as those are only for EE customers and non-commercial CE deployments (see Release Policy · OpenNebula/one Wiki · GitHub for more info).
To upgrade to the previous version (6.2), you can just use its latest CE Patch Release, which comes with the open sourced migrators already: CE Patch Release 6.2.0.1 is Available
Hope that helps 