I used openssl command line tool to generate a CA signed certificate. now I need to tamper it to generate the signature mismatch error. Is there any tool available to tamper the X509 certificate.

Open certificate file in ASN.1 Editor, find commonName node, right-click it and choose "Edit...": <img src="https://i.stack.imgur.com/c824s.png" alt="enter image description here"> Change the text of commonName field: <img src="https://i.stack.imgur.com/cnrQM.png" alt="enter image description here"> Hit OK, save your changes and there you are: <img src="https://i.stack.imgur.com/BRW0i.png" alt="enter image description here">