Many people worldwide want to be less neurotic. As lower levels of neuroticism predict a variety of positive life outcomes, including health, success, and relationship quality, interventions that help people to become less neurotic could have wide-ranging impacts. This study protocol describes the rationale and design of the CHILL (Changing How I Live Life) study, which aims to test and evaluate a smartphone-based intervention to reduce neuroticism in the general public. The CHILL study comprises four survey assessments spread over 5 months, a 6-week intervention phase, and two 1-week experience sampling phases without interventions. During the intervention phase, participants motivated to become less neurotic are randomized to four core treatment groups. These treatment groups differ in their employed intervention strategies that target different aspects and levels of neuroticism. The CHILL study moves beyond existing volitional personality change research by testing which strategies most effectively decrease neuroticism and evaluating the longevity and generalizability of intervention-induced neuroticism change via peer-reports, implicit personality assessments, and a 7-week follow-up period. The goals of the CHILL study are to generate an effective method for decreasing neuroticism and facilitating the development of a comprehensive theory of neuroticism change.