1, disable filevault.

2. Open the App Store and find the 10.9 system. Don't download here yet.

3. Then modify the system version number10.8 (/system/library/core services/system version. plist) and put systemversion.plist in front.

Copy it, and the two bits in 10. 10 are changed to 10.8. After modification, it is overwritten to the original location.

4. Go back to the apstore and start downloading the 10.9 system (I need a powerpoint to the network here). I downloaded it in 20 minutes, and then automatically jumped out of the installation, so it goes without saying.