Board Thread:Questions and Answers/@comment-33887487-20180704221307/@comment-27430351-20180711084417

You can only do it after neutral. No need to reset your game after your neutral run, (unless you gained any EXP or LOVE) you can just go back, hang out with Undyne, and do the True Lab part to get to the final boss of the Pacifist Route.

Hope I Helped!