reassure (one) about (something)
To make one feel confident or assured about something; to remove one's doubt or misgivings about something. I tried to reassure Sarah about her relationship with Tom, but I could tell she wasn't convinced. You'll need to reassure them about the security of their investment after such a disappointing fiscal quarter.
reassure someone about something
to give someone confidence about something. Nancy reassured Betty about her promotion. Betty was reassured about her promotion.