Dealing with measures and tempo in film music

Hello,
I don’t know how to do it the following thing in Cubase.
I have a range delimited by two markers. Behind the second marker there is a musical part that fits perfectly well. However, I would like to experiment with the part in between those two markers.

I would like to merge two measures into one, change tempos, etc without affecting the rest. How do you achieve such a thing?