Apple - iPhone 6 Boot loop and Error 9

iFixit has a solution to your problem.

The iPad in my experience is very picky about what cables it will restore with. Error 9 is almost always related to the cable, NOT the dock connector/flex cable. Use the official Apple cable if you still have it, or borrow one from a friend.

tl;dr: It's the cable's fault, plug your iPhone into iTunes and update your software again with a different cable.

This has happened to me before. Trying to update software on an old iPod Touch by connecting it to iTunes with a non-MFi cable, got an error message and a crash. Replacing the cable with a genuine Apple one fixed my issue.

If you're looking for a cheap, MFi-certified cable, try this $6 one from Anker. Or get the official $19 one.