If you wish to replace any supplementary map on the device you should make a backup of that first, if it exists, then simply copy and paste the supplied .img file in the same location (that will be either the unit's .System or Garmin folder depending on device). If you wish to have the OSM map on a microSD or SD card then place it in a Garmin folder named gmapsupp.img as that naming and placement will work for all Garmin devices with a card slot. Don't have more than one map of the same area selected at any one time. You may use MapInstall to create a custom sized map img from the .gmap folder if you wish.

If you need more info or assistance state which device you have.