IQAN 6 association with IQAN 5 files

I believe the last version you install is what opens on default. So, reinstall IQAN 5.

As both design 5 and design 6 files have the same ending (.idax) you can not really set a specific program to open only the design 5 files.

The closest is for you to set the default program to open all idax files to be design 5.

If you accidentally open the IQAN5 file in 6 and then close it, you will not be able to re-open it then in 5 which is a problem. 

Just opening and closing the file will not automatically update it, you have to save the file in Design 6 for it to be updated.

When you do save it Design 6 you will get a popup warning you if you try to save over an older version and also asking you if you want to create an backup of the old version. 

This backup will be located in the same folder as the original file an named (Backup copy (version x.xx) of name)

From IQANdesign 6.01, there is a feature update that solves this

Install one of the versions IQANdesign 6.01 to 6.08 and it should work. 

This associates the file extensions with a separate launcher application that opens the right program.