There is already a content DB. I want to change it to newly created DB. How to map newly created database to Nintex? Is it possible?
There is not such a thing as mapping one Nintex database to another Nintex database. Do you mean to move the content from the old database to the new database?
The maps that Alex Burton is referring to is great for mapping SharePoint individual content databases to individual Nintex Databases. This will however only apply going forward with new data generated by the workflows being placed in the new databases... it will not migrate the existing data from one database to another.
I just wanted to check that the previous answer worked for you.
As Alex Burton suggested in SharePoint Central Admin within the heading of Nintex Workflow Management there is an option for Database.
At the bottom of this window you have an option to create new database and another option to map databases.
If you need more information please ask or if everything is resolved please mark the question as answered using the Actions menu below the item which answered the question.