You could try & see if the Startup Manager can see the older OS X on the HDD:
How to choose a startup disk on your Mac - Apple Support
You may try & see if by starting up in OS X Recovery, you can use the OS X Utilities
and select a different startup volume (such as existing pre El Capitan OS X) from there.
OS X: About OS X Recovery - Apple Support
If there isn't an undamaged or existing original OS X version installed or available on
the computer hard drive, the OS X Recovery may choose to try & get your original
computer OS X. The older system should be there; but it may be damaged.
If you have a time machine backup for the system previously running in the Mac before
this failed upgrade attempt, you could use that to attempt to restore the former OS X.
Various good ideas and suggestions are covered here:
http://pondini.org/OSX/RecoveryHD.html
Usually you can attempt a startup and force the mac to look for any OS X installed by
use of keyboard command shortcuts. Startup shortcuts and others, are available.
Startup key combinations for Mac - Apple Support
Most other keyboard combinations for Mac:
Mac keyboard shortcuts - Apple Support
Hopefully these ideas may help. There are several possibilities so they may serve to
confuse; read round the issue/topic before attempting.
Good luck! 🙂