Do you have the Mendix Version Selector installed? As far as I know this should take care of it, as long as you have the appropriate version installed of course.
As far as I can see Mendix Version Selector is installed. In Program Files in map Mendix there is a Map Version Selector. When I run VersionSelector.exe, it shows a page: Manage Installed Modeller Versions. At this page there are 2 lines: First line: Version = 6.8.0, Type = Installed Second line: Version = 6.7.0, Type = Installed
Do I have to uninstall 6.7.0 ?
It should automatically detect the correct version. Does it fail to open the project in the modeler? As 6.7.0 should not be able to open a 6.8.0 project. If it correctly opens the project it is much more likely the project is actually a 6.7.0 project.
If it correctly opens, you can start Mendix 6.8.0 and manually open the project (File -> Open project -> select .mpr) and convert it to 6.8.0.