It would be great to be able to edit/revise recorded future absences rather than waiting for the day of to be able to make changes.
Future attendance can be edited on the Student Attendance page (Academics > Attendance > Student attendance.
Although not the intended spot to edit it, you can also see future attendance in the Add attendance for single student modal and make changes.
You say this "already exists," but it doesn't. If an absence is put in, and then something changes, you can't edit/delete that attendance entry until the day of.