Incredibly tough and convoluted!
Follow these steps very carefully!
1. Make backup copy of project.
2. Open RoboHelp 7.
3. Click File > Open and navigate to the project database.
You might need to click Tools > Update DHTML Effects in topics.
Helpful and Handy Links
The steps that Rick pointed out will suffice. Just be sure to take special note of step 1: Make sure you have a backup.
After opening the project with RoboHelp 7, click Tools > Update DHTML effects in topics. This will update drop-downs etc and perform various cleanups.
But before you do, make a backup of the X5 project. (Really, I can't say this enough.)