How re-encryption can be done through RSA
Both RSA and elGamal have homomorphic property. So both can re-encrypt the cipher text


That doesn't follow.

Re-encryption means being able to take $E_{a}(m)$ and $F(a, b)$ (that is, a function of both private keys), and being able to generate $E_{b}(m)$ (that is, the encryption of the same message under a different private key), but (and this is the critical part) it not being able to recover $m$ from those two.  That is, $F(a, b)$ is a key that allows you to "switch keys", but is not sufficient (without the private keys $a$ or $b$) to allow you to decrypt.

What the homomorphic property of RSA is allow you to take $E_{a}(m)$ (and the public key), and allow you to generate $E_{a}(G(m))$ (for certain functions $G$).

That doesn't allow you to switch RSA keys (which would imply changing modulii), and there is no obvious way to do so...