I haven't found a duplicate which explains with sufficient clarity how does the process of rotating encryption keys work in simple layman terms.

I know how private/public key cryptography works, I know how symmetric cryptography works but I can't understand how *rotating a private key* works.

Do I decrypt all of the encrypted content, generate a new private key and then re-encrypt everything? Or perhaps do I generate a new private key and decrypt and encrypt with the new key just part of the data so now I need two keys (the new and the old) to decrypt the whole?