How to access the crm files
I would recomment reading up on how to customize CiviCRM here: https://docs.civicrm.org/dev/en/latest/ and here: https://docs.civicrm.org/sysadmin/en/latest/customize/extensions/

That should answer your question and also allow you to do the customizations in a recommended and structured way.

If you would like a more specific answer it will help us all ot help you if you tell us a little more about what you want to change for what reason, what version of CiviCRM you are using and what CMS.