how to reset the paraphrase for an account on private ethereum network
I don't find this question very clear. Deep down, accounts are addresses with public and private keypairs. Either you have the private key or you don't. Local copies of the private key are stored in encrypted format (so keys are not available in plain text). In theory, the encryption password could be changed because Ethereum is indifferent to exactly how the user recovered the private key.
HD Wallet schemes use mnemonics to generate public/private keys. You couldn't change that and you couldn't change the address or the keys used by an address. In case it is not clear, the situation is analogous to certificate-based systems, and either you have the secret or you don't, but you can't change it. It's not that the user "has" an address and a secret. The user "is" the address and a secret.
You can build systems of virtual identifiers with interchangeable signing delegates. That might be worth exploring if you are looking for a very powerful user management system.
Is there a way to delete an account on private ethereum network
Not really. It's an immutable system. All address "exist" but a user only knows the signing key for some addresses. History doesn't go away, so what is meant by "delete" is an application layer concern. For example, a contract may revoke privileges extended to a particular address, e.g. 0x123 is not the admin anymore. This is generally more practical than trying to prove that the user has destroyed their private key (and all copies of it).
Again, this depends on precisely what you mean by "account". There are advanced patterns that use delegates to sign on behalf of virtual "accounts". Such patterns attempt to deal with multiple devices or personnel acting "on behalf of" an entity as well as edge cases such as lost/compromised signing key.
How to enable the etherscan url for Private Ethereum network
I don't think this is possible. A "private" network is, by definition, not accessible to a service like etherscan. Indeed, confidentiality is often a concern and a reason for selecting a private network. It would be opposite of that if a public block explorer could expose everything. On a system like Quorum (Enterprise Ethereum Alliance) the private states are not even known to all nodes so there is no vantage point from which everything is observable.
Block explorers "like" Etherscan can be deployed internally. Several are available as open-source tools you can deploy to give your private network a block explorer. I would caution that the mere act of doing so might compromise confidentiality and there will be special challenges if your private network uses Quorum-style private transactions.
Hope it helps.