Have you already parsed t and y from the text file?
I would suggest custom nonlinear fitting since it looks like you want to fit an exponential curve with a constant additive term. Here's a doc page about using CFTOOL to do this.
For example, for the function you are using, I would define a fittype like this:
f = fittype( @(a, b, C, t) a*exp(-b*t) + C);
coeffs = fit(t, y, f);
a = coeffs.a;
b = coeffs.b;
C = coeffs.C;
Make sure that t and y are column vectors. Good luck!
