Hey there, how do uninstall El Capitan and downgrade back to Yosemite?
Downgrade to Yosemite.
Best way is to download Yosemite installer and use the instructions to build a USB bootable installer.
https://support.apple.com/en-gb/HT201372
If you have a time-machine backup, then you should be able to manually mount the image, and restore the individual files from there.
Keep in mind that certain files will be upgraded to newer versions, so may not restore.