Most common cause of the symptom
You are not a Project Admin. Only Project Admins have the ability to update project details. If you have not been made Admin on a Project which you are a part of then you will not see the ability to update the details, instead, you will be presented with the project details in 'Read only' mode.
Our permissions system only allows Project Admins to update the details as you can add many people to a Project and not everyone should have the ability to edit those details.
1. Please contact your Project Admin to ask them to give you Project Admin role.
Note: External Users cannot be made Project Admins so you must also be a part of the Plan to which the Project is attached. See Add plan users.