OP:
Some advice.
First, if the "Office issue" is the only one facing you, it might be worth staying where you are.
But -- if you
absolutely, positively want to "go back"...
First you need a backup of your data as it is now.
I would HIGHLY RECOMMEND that you download CarbonCopyCloner from here:
http://www.bombich.com/download.html
CCC is FREE to download and it's FREE to use for 30 days.
Use CCC to create a cloned backup of your current internal drive
This will be
INVALUABLE to you if you need to restore your data bit-by-bit later on.
AFTER THAT:
Get the El Cap installer from the link in reply 4 above.
DON'T attempt to run it.
Instead, do this:
1. Get a USB flashdrive. I suggest 16gb, but 8gb -might work- if you try it.
2. Download the free app "Boot Buddy":
https://sqwarq.com/boot-buddy/
3. Use BB to create a BOOTABLE USB flashdrive installer of El Cap
NEXT:
1. BE SURE YOU ARE BACKED UP FIRST USING CCC
2. Boot the Mac using the USB flashdrive installer by rebooting and holding down the option key until the startup manager appears. Then select the flashdrive and hit return
3. You should boot to the installer, but -- DON"T RUN IT YET
4. Quit the installer and open Disk Utility
5. ERASE the internal drive to HFS+ with journaling enabled
6. NOW quit DU and open the installer again
7. Run the El Cap installer.
8. When the installer finishes, you'll get the setup assistant.
9. At this point you can begin setup assistant. When it asks if you wish to migrate data, you can TRY to connect the cloned backup and see if SA will "bring any of that over".
It might work, but MIGHT NOT, because you are trying to go from "newer to older".
DON'T DESPAIR.
There are other ways to "get your data moved back".
IF you try the above and get this far, report back, and we'll tell you what to do next.