Don't flip your bike over unless you want scuffed saddle and handlebars/brifters.

If you DO have to loosen the fastening bolt on the cable, make sure to do it in the smallest cog/chainring. This is where you won't have any tension on the cable and it will be easy to re-fasten it. It is uncommon to need to re-fasten a cable in a tighter position if it was indeed set up correctly. So-called "cable stretch" (actually just seating/wearing of the various housing/cable ends) is minor and can almost always be compensated by the adjuster barrel. You should be very certain something else isn't wrong before you do this.