To update your mobile phone number:

• you may visit the User Support Office of the Digital Governance Unit (DGU) in person, presenting an official identification document (e.g. passport). A third party may also visit on your behalf, provided they have proper authorization,
• alternatively, you may contact the administrative unit responsible for maintaining the registry of members in your category (e.g. for faculty members, the Department of Teaching Staff is responsible; for students, the Secretariat of the respective School), and follow the instructions provided to you.

Greek citizens should follow the instructions here.