I'm trying to compare two nonlinear curve fit models using an F test. Why can't I move back and forth between Equations 1 and 2?

You may have encountered a bug in Prism version 3.02, whereby the radio buttons get stuck with both equations selected:

Once you've set Equation 1, you can't modify Equation 2. This was fixed in version 3.03, but you can also work around the problem by selecting Equation 2 first, then Equation 1. If you've already gotten stuck, just cancel out of the analysis and start it again.