Choose one of the similes in "Harlem" and explain how it helps the reader understand what happens to a deferred dream.
I wonder if you have perhaps become slightly confused between literary terms in this question. Unfortunately, this excellent poem by Langston Hughes contains no similes. However, it does contain metaphors, and I wonder if this is what your question is referring to. Similes and metaphors are very similar in likening an object, place or character to something else, but similes use the words "like" or "as" to do this, whereas metaphors assert the comparison directly.
Note how this operates in a repeated metaphor in this short poem:
So we stand here
On the edge of hell
Here, a metaphor is used to compare Harlem to "the edge of hell," obviously relating the impoverishment and lack of rights that Afro-Americans were experiencing at the time with hell. I suppose the "deferred dream" that your question refers to is the dream of what such Afro-Americans would like or imagine America to look like in terms of racial equality. Their imaginings of a world free from racism, where Afro-Americans are able to get jobs in spite of the colour of their skin, is a dream that is necessarily deferred because of the immense prejudice and racism that existed at the time, which is reinforced through the metaphor analysed above.