Learning assignment - how to do a "edit assignment" massively

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Hi all,

from My Client Group > Learning > Learning assignment, we would like to change assignment type from "required" to "voluntary". Selecting only one row we can do the change, but when we select more rows the action "edit assignment" disappear, as you can see in the screen below.

Is there a way to mass edit assignment? Is any privileges missing?

Thank you,

Alessandro.