- Create the new content database ( Using Central Admin or PowerShell ) on the same farm and web application.
- move the site to the newly created content database in step 1 by using the following PowerShell :
Move-SPSite http://servername/sites/sitename -DestinationDatabase NewContentDatabaseName
please note during this process the databases will be locked and users will not be able to modify the site ( add modify documents and lists ) you need to run this command with Admin rights.
- Reset IIS on each front end.
- Release Old content DB storage by starting the timer job “Gradual Site Delete“. you can use central administration –> monitoring –> Job definitions -> run now