You probably can't.
I've been down this road before. Last place I worked, the VP came around and said "the CTO says we have to fix all bugs within 90 days." As we did scrum and I had a backlog and team velocity, I did some quick calculations for my team and the other directors' teams, and said "At the current rate of clearing bugs with the current team that will take months just to clear the backlog and after that it'll be 80% of our dev capacity on ongoing bug fix. Are any of the product deliverables moving?" "No..." "Well then it can't be done, not and hit those." "But... The CTO said..." So we prioritized bugs slightly more, but also had to work to hit deliverables, and even with people doing the usual "well but is it really a bug" shenanigans we fell way below the bar. For a couple months, the VP kept asking for updates on it. We'd tell him and remind him about the team's capacity. Eventually he stopped asking because the CTO largely forgot he had tossed that edict out one day.
So that is to say you can try, you can use metrics and projections to indicate what the team's capacity is and what they can do per unit time, and you can certainly propose improvements that would reduce bugs/speed fixes/whatnot, and/ir ask for more people, but in the end most of not all organizations are not metrics/data/reality driven, they are emotion and snap decision and politics driven.
Focus on doing the work and continually improving. Show graphs of your fix rate creeping up. Then play the politics game of tooting that horn, saying "we could do more if only X", and then at the right time, strategic silence to let upper management save face by letting the edict slip into the night unremarked.