Select Assignments from the site's Tool Menu.
Select Permissions from the Actions Menu.
Modify the permissions for the roles listed.
Check the box next to each permission to allow for a given role. (Unchecking a box will remove the corresponding permission.)
Note: You may see different roles listed depending on the roles which exist in your site. A role must be present in the site in order for you to modify its permissions.
Select Save to save changes.