Most Macros should probably be Application Macros. Application Macros are available to use in every project opened from the Revit Install that they were created in. Project Macros are only available in the project they were created in. If you want to copy a project macro to a new project, a quick web search should give you a few options.
But you may want to use your Application macros on another computer. Or worse, you may develop several macros, and then upgrade to a newer version of revit, only to find out you don't have those macros in the newer revit version. Here is a way to copy them. I discovered it quite by accident.
Under the ProgramData folder, search for a folder called "Macros". Search in that folder until you find a folder with your Macro's Module name. (Remember, macros are stored in modules, as shown in the Macro Manager window.) You want to copy and paste that folder into the new version of revit.
For example, to update a module called "CopyElements" from 2017 to 2018, you should be able to find and copy the following folder:
C:\ProgramData\Autodesk\Revit\Macros\2017\Revit\AppHookup\CopyElements
to:
C:\ProgramData\Autodesk\Revit\Macros\2018\Revit\AppHookup
Copy the folder for each macro module you want to copy. Then restart the new version of Revit, and open the macro manager. You will be disappointed to find out that the copied macros don't show up. But fear not, there is one more step.
Create a new module (with the same program language), and give it the same name as the old module. If the edit window doesn't open automatically, open it up, and surprise, all your code is there. I got an error message about a missing file on the first build, but everything seemed to compile and run. And after closing and reopening the macro manager, everything seemed to be OK.
If you are upgrading to a newer version of revit, you may also have to upgrade your macro to a newer version of C#. You can do this by selecting "Tools, Project Upgrade" in the Sharp editor. For more info, see the Revit Help under "Customizing Revit - Automating Tasks with Macros - Upgrade Revit Macros"