Are you doing this inside of the Photos app? I use Photos, and giving a bunch of pictures the same location is pretty easy in there. My camera reads the location from my iPhone by bluetooth, but sometimes I forget to turn it on, so I do this all the time. In Photos, you select the pictures, open the info window, and type in the location. The location gets assigned to all the pictures at once.
You can enter a location by name, Grand Canyon or Eiffel Tower, for instance, and the latitude and longitude for that place get entered for the picture. Photos uses a lookup table to show the name of the place. You can enter the latitude and longitude of the Eiffel Tower if you like, and it will show "Eiffel Tower." You can also do a generic Dallas, Texas, and you will see the location of City Hall.
If you already have the location entered for one picture, then you can use the menu Image>Location>Copy location then use Image>Location>Assign location to a bunch of other pictures.
Outside of Photos I use GraphicConverter ($40) which does lots of useful stuff. But for location, Photos is easier. For location specifically, I have used the app GeoTag, but with Photos being convenient I haven't used it for years. There are a number of apps available for metadata and location editing, so you could check the app store. But don't try to use a files editor like GraphicConverter or GeoTag on pictures loaded into Photos-- it can really mess up the database!