You can improve your VO2 max with regular aerobic exercise that raises your heart rate—the more vigorous the work, the stronger the stimulus. Walking can be enough if you are new to exercise; more fit people usually need harder efforts such as running, cycling, or swimming.
High-intensity interval training (HIIT) several times per week, mixed with longer, easier sessions, is especially effective because short hard efforts push oxygen use toward its limit. Varying your routine, extending workouts when appropriate, and maintaining a healthy weight can also help, since relative VO2 max is measured per kilogram of body weight. (Harvard Health)