Are you looking to have "a" change within a single call to ODE45, or would you like it to change on each subsequent call?
If the latter is the case, you could use an anonymous function wrapper to the derivative function
function dy = derivs(t,y,a)
dy = a*y;
And then
>> derivWrapper = @(t,y) derivs(t,y,5);
>> [t,y] = ode45(derivWrapper,[0 10],2)
>> plot(t,y)
If the former is the case, you could modify your derivative function to change "a" as you want to.
function dy = derivs(t,y)
if (t > 5)
a = 1;
else
a = 3;
end
dy = a*y;
