Access the dialog by clicking or tapping on a requirement slot.
This dialog will allow you to change the requirement for that slot, and also optionally allow you to pre-assign a guild member to that slot.
The close 'x' button or the escape key or clicking/tapping on the requirement slot again can be used to close the dialog.
The first dropdown includes a list of all of the possible collectibles for the parent platoon. Choosing one will populate the picture in that slot and will increase the overall mission's progress bar.
The second dropdown includes a list of all guild members. Choosing one will darken the corresponding requirement image and display who was chosen. For more information about pre-assigning guild members, see this article: