Here are instructions that helped me with Downgrading windows 8.1 to windows 7
1. Put in your window 7 CD in the CD/DVD rom
2. Turn on your computer for Windows 8
3. Shift + Restart
4. Click on Troubleshooting
5. Click Advanced Options
6. Click UEFI Firmware Settings
7. Click Restart
8. Then your computer will reboot and take you into your BIOS
9. On the BIOS go to Security
10. Go down to Secure Boot then Disabled
11. Then go to Advanced, then to System Configuration
12. Then go down to Boot Mode
13. Change it to CSM Boot
14. Then go to Boot and change the order of the Boot to your CD/DVD Rom first
15. Follow process to install, click on custom advance
16. Delete all partitions by clicking on driver advance options then delete
17. Only one partition should be left,click on new then apply
18. Click ok then next
Hope this helps.