Hi @mojgan_parizi,
I believe that by default, users with the designer role can edit assignments, which I think is what you're asking about. Permissions can be customized by every school/institution though, so it's hard for us in the community to know how things are set up for your Canvas instance as we cannot access it. This may be a question to ask your local Canvas administrators, usually located in the IT / eLearning / Digital Education areas of a school/institution.
Hope this helps a bit.
-Chris
