How can one turn a malleable encryption to not malleable
It seems like you are asking about converting an IND-CPA encryption scheme into an IND-CCA one.


In the symmetric-key setting, the standard approach that works is Encrypt-then-MAC.
In the public-key setting, this is a longstanding open question. There is no known black-box construction of IND-CCA from IND-CPA, nor is there a known impossibility of such a construction. However, there is such a transformation in the random oracle model: the Fujisaki-Okamoto transformation.