Can't upgrade from mountin lion to yasimite

Question

can't upgrade from mountin lion to Yosemite ?

Answer 1

Have you tried:
Repair the Hard Drive and Permissions - Yosemite, Mavericks, Mountain Lion or Lion
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Repair
When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the Macintosh HD entry from the the left side list.  Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit Disk Utility and return to the main menu. Select Restart from the Apple menu.
Now, re-run the Yosemite installer application or re-download it if you don't have it yet. If you are getting server errors, then you will need to check your internet connection and service or wait to try another time.