This problem is of Fernando Vega Redondo(Economics and the theory of games)
Exercise 2.1 Let G be a game in strategic form. Prove that, for every player $i\in N$, every mixted strategy $\sigma_{i}\in \Sigma_{i}$ that assigns positive weight to a pure strategy $s_{i}\in S_{i}$ that is dominated can be itself always be improved by another strategy $\sigma_{i}'$.
This is if $s_{i}\in S_{i}$ is strongly dominated for some $\sigma_{i}$$\in \Sigma$ with $\sigma_{i}(s_{i})>0$, $\exists$ $\sigma_{i}'\in \Sigma_{i}$ such that $\forall$$s_{-i}\in S_{-i}$$: \pi_{i}(\sigma_{i}',s_{-i})>\pi_{i}(\sigma_{i},s_{-i})$.
Q1: why they said that affirmation is obvious?
*I would like to know if anyone could tell me how to build that mixed strategy that dominates that strategy that assigns positive probability, since it is not obvious to me.
Thanks!