How and why does Sheila change in 'An Inspector Calls'?

In Act II, we can see that Sheila clearly has begun to change. She is owning up to her responsibility for Eva’s death, maturing as she does so. Notably, she stands in stark contrast to her mother, who refuses to change at all and (so far) refuses to drop her mask of icy, upper-class politeness. Priestley is interested in the well-worn idea that the young have the capacity to change, accept new ideas and move forwards while their parents and the older generations often fail to do so.

Shortly before his exit, we see that Sheila similarly has the maturity to, without tears, accept that things are now different between her and Gerald, even unemotionally offering the symbolic gesture of the return of his ring. Maturely, she accepts her part of the responsibility for Eva’s death, noting that it is better that “at least [Gerald has] been honest.” Moreover, as she points out to him, “this has made a difference,” and the engagement will not be able to continue without serious reconsideration.

The moment when Sheila returns Gerald’s ring perhaps symbolizes the distance the play itself has come: its comfortable “engagement party” opening has been entirely turned on its head.