The File Manager is enabled by default in cPanel. You need to login into the cPanel account first before you can use it.
How to access the File manager: https://documentation.cpanel.net/display/68Docs/File+Manager
Depending on your service, in case you don't see that option, you are probably signing in through WHM - which is the administration part of cPanel.
When you log in via WHM, you need to click on List Accounts and then click on the cPanel Logo next to the domain name to access the cPanel of that account.
If you don't see that option, make sure it is enabled in the Feature Manager: https://documentation.cpanel.net/display/64Docs/Feature+Manager
Make sure the package
of the cPanel account
is using the same Feature List