As you build your game, Unity stores a lot of metadata about your assets, such as import settings and links to other assets, among other information. If you want to transfer your assets into a different project and preserve all this information, you can export your assets as a Custom Package.
See Packages for detailed information on using Asset packages, including importing and exporting.
Use Export Package to create your own Custom Package.
HINT: When exporting a package Unity can export all dependencies as well. So, for example, if you select a Scene and export a package with all dependencies, then all models, textures and other assets that appear in the scene will be exported as well. This can be a quick way of exporting a bunch of assets without manually locating them all.
Sometimes you may want to change the contents of a package and create a newer, updated version of your asset package. To do this:
Select the asset files you want in your package (select both the unchanged ones and the new ones).
Export the files as described above in Export Package, above.
NOTE: You can re-name an updated package and Unity will recognise it as an update, so you can use incremental naming, for example: MyAssetPackageVer1, MyAssetPackageVer2.
HINT: It is not good practise to remove files from packages and then replace them with the same name: Unity will recognise them as different and possibly conflicting files and so display a warning symbol when they are imported. If you have removed a file and then decide to replace it, it is better to give it a different, but related name to the original.