How to change the password that open a PDF file?
Ask the owner of the file for the password
You can't change it in Reader. You would need Acrobat for that.
In the paid version of Acrobat, you can do that in the Security tab of Document Properties. You need the original password to remove security, then you click the checkbox for a password for security to open the document and enter the new password.
Note that this only works if you have the current password to open the document, plus if there is a second password to prevent edits, then you will need that to be able to remove the security.
Password kaise change karna hai