If you've not left it too long then, hopefully, the slip rings won't have burnt away you can change the brushes from the top.
If folk would only change the brushes as soon as the red light starts to flicker most alternators could be saved. Once that light begins to flicker the damage occurs faster and faster.---
No work under the vehicle required unless you need to change the alternator completely. Even then, that can be done from the top with a bit of fiddling about under the wheel arch.