If you don't want to reformat (and you aren't re-using a password you've used elsewhere) you can change the password: 1. Open Disk Utility and select the volum

If you don't want to reformat (and you aren't re-using a password you've used elsewhere) you can change the password: 1. Open Disk Utility and select the volume 2. Select File, Change Password 3. IMPORTANT: If on High Sierra GM, leave the hint field blank. If you've installed the update then you can provide a new hint. Word of warning: the most secure course of action is to follow the instructions in the linked support article.