How to change linear cooling rate to non-linear in nucleation experiment code?

What I'm trying to do is adjust the code so that the cooling rate starts at 6 K/h and then after a fixed time say for instance 1000s the cooling rate then switches to 12K/h and so on. So it would be case of one line being plotted on the graph with a step change after 1000s. Attached is the code and the relavant excel files for said code. Currently I just have multiple linear cooling rates.

Thank you!