You can't change what's already there, only what appears from now on. If you want to get rid of everything in maps, go to Settings/Apps/Maps and clear cache and data. But if you want to save some locations, don't do that. (Or install
MyDirections [it's old, hasn't been updated in a long time, doesn't read what it backs up sometimes - but it's handy], and put the ones you want in there [you have to be at the location when you add it in order for it to work - another thing that was "updated" to unusability].)