With epoxy-based putty or paste. Take a tool and dig out as much rot as you can. There’s a two part epoxy you can get that goes on either as a paste or a putty.
For the paste, you’ll need to build it up over a few coatings. For the putty, you can form it like play-doh.
Use batons to form it or get it in rough shape and then use planers, very sharp chisels and/or sandpaper to shape it. I typical do the shaping when the epoxy is not 100% cured but rather, 60%-80%. Easier to shape.
Once in the correct shape, allow to cure and paint.
Don’t use liquid epoxy. Unless you have a 100% fool-proof seal, you’ll make a mess. Hell, you’ll probably make a mess even with one.