I don't think you will need any such kind of correction. Your 12th certificate is not recognized as your signature. The problem of your signature occurs and is needed during financial activities like banks etc. When you do any transaction with the bank, then your signature should be the same. For this, your signature already registered in the bank is matched, not the signature done in your 12th mark sheet.
Even when filling other forms or applying for any job, when there is a demand for various identity cards and certificates etc. for identification, the school certificate is only needed for identifying your age and your parent's name, for other things like home address, etc. different identity cards are used.
If you have forgotten the signature that you had made while opening an account with the bank, then in such a situation you can go directly to the bank and contact, with a systematic process you can change your signature.
However, if you want to get your certificate corrections, then for this you will have to consult your school and the concerned board from where you passed your 12th examination. There you will have to follow the whole process, for which you have to fill a form and submit the relevant documents. In this, the details of your bank account can also be asked, in which you have your signature.