It depends. You can't rename premade folders that come with C4D, such as Visualize, Studio, Prime, etc.
I'm not sure if you can rename third party libraries not made by you or MAXON, but you CAN rename your own libraries: just rightclick and untick "Read Only". After that you can rename, change the contents (such as adding or deleting stuff). Don't forget to retick "Read Only" afterwards to protect the folder from accidental changes. Depending on the size of the folder this process could take a couple of minutes.
DO NOT close Cinema while it appears to be "frozen"! If you do while it's locking the folder you can potentially corrupt it!