Upgrade from 10.9.5 to 10.10.* without App Store

Get someone to download the installer from the App Store - just download it, don't install.

Apple documents how to create a bootable installer - https://support.apple.com/en-us/HT201372 using any USB drive or even an SD card should your Mac have that slot.

Plug that device in to your Mac.
Remember to hold down the option key when your Mac starts up.
Select the device.

One last thing, make sure you check your backup before installing. Time Machine is best. The upgrade isn't designed to have problems, but you never know.

I'd really consider trying to fix the existing OS long before exacerbating the situation by trying to force another over it.

Start with Disk Utility - Repair Disk Permissions & Verify Disk [startup from the Recovery partition, Cmd ⌘ R at the startup chime, if the Disk needs Repairing].
Again from the recovery partition, reset home folder permissions & ACLs, as per this answer - https://apple.stackexchange.com/a/163449/85275
- Steps 1 & 2 can instead be performed using the freeware Onyx utility…

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