\(u=2-x\Rightarrow x=2-u\Rightarrow du=-dx\)
\(\Rightarrow-\int\left(2-u\right)^2.u^{2020}du=-\int\left(4-4u+u^2\right).u^{2020}du=-\int\left(4u^{2020}-4u^{2021}+u^{2022}\right)du\)
\(=-\dfrac{4}{2021}.u^{2021}+\dfrac{4}{2022}u^{2022}-\dfrac{1}{2023}u^{2023}=...\)