Question Need a 5 minute 3D modelling task help/

I have purchased 2 3d model assets. This one and this one.
I have combined them to look like below.



Because the city is flat and the farm terrain has small hills I run into problems like below.



Basically I need a quick model to tapper off these hills into a flat area so I can use the city properly.
The roads from the flat city don't line up with the raised hill area.

I'm happy to pay a small payment if it's more work then I expect but I'm hoping an experienced modeller will find this easy. Please let me know if anyone can help.

Thanks