stereochem of epoxide opening
In these three-membered ring reactions, it's an SN2 substitution reaction that happens. Hence, there is an inversion of the configuration associated.
Edit: Perhaps the mechanism is more complicated than SN2, contrary to Klein's book which states that it's SN2, this online source says that the actual mechanism is between sn1 and SN2 (leaning to SN2 side) to explain the preference of nucleophile to attack tertiary carbon though more sterically hindered.

Reference: See page-431 of Organic Chemistry David-R-Klein