After creating a project role, you can always go back to editing it again. For single project roles this is done by clicking on the ellipsis icon and choosing the "Edit project role" option.
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You can also mass-edit project roles in Silverbucket in views, that list multiple project roles for you at once: project's planning view and the main views for people and projects for example.
Select your project roles and choose the "Mass edit project roles" under the ellipsis icon located just top of the listed roles. If you wish to select all, you can first click on the ellipsis icon to select all and then click it again to perform a mass-edit. Careful with this, as this will affect every single project role listed below.
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In the opening view you are free to edit data for all selected project roles.
Here we are setting the role for all three roles as "Construction Engineer" and simultaneously setting those roles to have an hourly rate of 90