1. Navigate to the subproject.
2. Next to "Subproject details", click "Edit".
3. Change the "parent project" setting of the subproject.
4. If you select a client here, the project will become standalone. If you select a project, the subproject will be migrated to become a subproject of the selected project.
NOTE: This only works if your account is configured to log time to Clients & Projects, or to Projects through Client selection. This configuration is found under Settings > General Settings > Time & Projects. If your account is configured to log time to projects only, you can change this setting temporarily to allow the subproject upgrading as described above.