In your team's admin area, you can rename projects using the Edit action, which you can find in multiple places, such as the All Projects page or the Projects Overview report.
In Slack, you can use the following command to quickly edit a project name:
rename project <current project name>, <new project name>
Admins can edit any project. Depending on the Roles and Permissions option in Team Settings, non-admins may or may not be able to edit the projects they worked on.