You also need a copy of LabVIEW 7.0.
First you convert the 7.1 code to 7.0 using "file...save with options...save for previous". Now you open it in LabVIEW 7.0 and repeat the same, saving it for 6.1.
If your 7.1 program uses features that were not available in 6.1, you might get a broken vi which you need to fix up manually.