How does Benedick change over the course of the play Much Ado About Nothing?A descriptive answer would be helpful.

I changed the reference in your question from "how your
opinion of Benedick changes" to "how Benedick changes."  Opinions are as changeable as
the weather and endless in their possibilities.  Let's look instead at how the character
seems to transform throughout the events of the play.

At
the opening of the play, it seems that Benedick will serve throughout the play as the
butt of everyone's jokes, especially Beatrice's jokes.  No matter how he tries, he
cannot get the best of her.  Not in the opening scene of Act I, nor in the dance scene
(Act I, scene i), when he believes that he has disguised himself so thoroughly that
Beatrice will have no idea who he is. She knows him instantly.  The audience sees only
the humour associated with Benedick in these early
scenes.

In Act II, scene iii, he sets himself up to be the
further butt of jokes when he has a long soliloquy in which he makes funs of Claudio for
falling in love and marrying, and he swears that such a fate would never befall him. 
And, in the action that follows, he is duped by Claudio, Leonato and Don Pedro into
believing that Beatrice loves him.   Pratfalls and his innuendo-ridden conversation with
Beatrice follow.  Benedick swears himself now in love with Beatrice, but seems to still
simply be an amusing character, one made even more riduculous in that he has just given
in to the one thing he said he would never do -- fall in
love.

Things change after the aborted wedding in Act IV,
scene i.  Benedick is left alone onstage with Beatrice (their first conversation
together since declaring their love -- to the audience -- for each other).  It is a
deeply serious scene, one in which Beatrice reveals her raw inner pain about her sex. 
Since she is a woman, she cannot fight for her cousin's honor.  She commands Benedick to
"Kill Claudio."  She demands that Benedick do this to prove his love for her.  And,
though Claudio is his friend, Benedick agrees.

The serious,
grown-up side of Benedick doesn't end here.  When he meets up with Claudio and Don Pedro
in Act V, scene i, he brushes off their attempts at lightheartedness and, in a gallant
and manly way, challenges Claudio to a duel.  He
says:

You are
a villain.  I jest not; I will make it good how you dare, with what you dare, and when
you dare.  Do me right, or I will protest your cowardice.. . .Let me hear from
you.

This Benedick is every
inch a hero and ticks all the boxes of the Romantic Comic Hero, just as Claudio, who
seemed to fit the heroic bill in the play's beginning, now seems a buffoon. The tables
have turned in a surprising way.

Throughout the rest of the
play, Benedick behaves in a sober and heroic way, graciously pardoning his friends at
the end and marrying the woman he loves.  So, throughout the course of the play,
Benedick transform from clown to hero.

Please follow the
links below for more on the character of Benedick.