Plot the temperatures against time on a graph, like below. Then obtain and plot the best-fit lines (linear, quadratic, cubic). See below (I've not plotted the cubic below because it obscures the quadratic). You should really spend time learning about curve-fitting and the statistics of curve fitting - see http://en.wikipedia.org/wiki/Curve_fitting for starters, for example.
CurveFits.PNG
Incidentally, this is all linear regression not non-linear regression.
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