How can I change this explicit method code to the implicit method code?

f = @(t,y) -100000*y + 99999*exp(-t);

t0 = 0; y0 =0;

h = 0.01; tn = 2;

n = (tn-t0)/h;

t(1)=t0; y(1)=y0;

for i=1:n

t(i+1) = t(i) + h;

y(i+1) = y(i) + (f(t(i),y(i))*h);

end

plot(t,y)