Repair of mortar joints, called tuckpointing, is a process that removes the outer one inch of mortar from the joint and replaces it with fresh mortar.  There is a huge learning curve to becoming proficient at tuckpointing.  We would recommend that you do not take on tuckpointing unless you’ve been trained in its nuances.  Over the years (31 years at the time of writing) we’ve seen many DIY tuckpointing jobs that actually ruined the aesthetics of buildings and homes.