Best way to lower pipe for boxing in

[BathDIY]

I want to lower the uppermost pipe in the attached images so I may more easily box it in.

The soil pipe is internal am assuming it's a matter of a direct connection to the soil pipe somehow although am guessing it's cramped or why else would this pipe be so high?

 

[cross thread]

As long as it has a fall on it from the sink to the outlet it can be lowered