1. Go to your project, and click create > new prefab
2. Drag the projectile you have in your heirarcy onto the new prefab
3. Rename the prefab as you like
4. Drag the prefab from the project folder onto the variable in your script that was previously pointing to the projectile in the hierarchy
5. Delete the projectile that was in the hierarchy

Following these steps will create (and link to) a prefab in the project folder that can be pointed to like any object in the scene/heirarchy