Yes you need it to install software and updates. You can change it this way:
Right after you hear the chime, press and hold the Command and the R keys for a few seconds, then release. Soon, you'll see the Apple logo and spinning gear (or progress bar in Yosemite).
This will force the Mac to boot into OS X on the Recovery Partition that was created when you installed OS X 10.7 or later.
3. When booting is complete, you'll see a window of "OS X Utilities." You won't do anything there. Instead, you'll go to the Menu bar at the top and select Utilities > Terminal.
Don't worry. You won't have to do anything exotic on the Unix command line.
4. In the terminal window type the following and hit Return.
5. A new window will open, like this:
Click on the 1) volume of interest, 2) select the desired user account name in the popup. Then enter the new password (twice). A password ***** is optional. Click "Save."
6. Go back to the Apple Menu at the top. Select OS X Utilities > Quit OS X Utilities. That will prompt you to restart the Mac.
If all went well, when the Mac restarts normally, you'll be able to log in to the administrator account with the new password ***** defined in step #5 above. From there you can manage the rest of of the users if necessary with System Preferences > Users & Groups.