... I cound not find any hints in the user documentation.
As long as you have "edit" permission in the project, and the "Description" field is on the edit screen for the project, then you should be able to edit the description.
If the bug is closed, then it is also possible that the person who set up your workflow has made it uneditable (in the properties of the step on the workflow). If that is the case, then you will need to 're-open' the issue in order to edit it.
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