Very simple: you download, rename, edit, upload:
#1: Download calc you want to use as template
#2: Go into calc and tap Edit
#3: Tap on name and change to whatever name you want
#3: Mod to your heart's delight
#4: Scroll to the bottom of the definition, tap Sharing and then Upload
Done. (Note, there's no upload confirmation. It just takes a second and it's done. If an error occurs (for example, you don't have a network connection), you'll get an error dialog.)
In the steps above, you don't have to rename. When you upload, you upload into your own personal area. That is, you're not overriding the shared calc you used as base. Whenever you download, you'll download your copy, not the common/shared version.
If you didn't rename and want to go back to the common/shared version, delete your local calc of the same name. (Swipe left on it in Calculators.) Now re-download from Downloadables.
Hint: if you uploaded a calc of your own and you accidentally deleted it, not all is lost. Just create a new calc of the exact same name and then go to Sharing and tap Download. That will repopulate your calc's definition from your last save.