Hi Ivan,
Currently, we don't expose an endpoint to change the admin's login name. The current endpoint exposed only allows you to change some details for that user such as their first and last name, and email. We have another method that can be used to create a new admin user (https://docs.q2api.com/docs/q2-caliper-api/40aa58e1c9690-add-admin-user). You should be able to disable your current one wtih the disable an admin endpoint (https://docs.q2api.com/docs/q2-caliper-api/328d5e15e68bc-disable-an-admin) and create a new one with the preferred login.
Best,
Solène