The project link is an unique identifier for the project within the workspace. If you don't enter a link while creating a project, it will be automatically created from the project name. If the chosen project link is already in use within the organization, you will be prompted to choose a different one.
Editing the project link after project creation
Click on Projects on the left side pane.
Click on your project.
Click on the Settings tab. If you have more than three tabs enabled, then Settings will appear under the More tab.
Click on the Project info tab.
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A right side pane will appear and you can update the URL as needed.
To know more about this, refer to this help article.