。
Can I change the return date?
Literal
Return-date [object-を] change can [question-か]?
変更できます is the potential form of 変更する using the ~できる shortcut — 'is able to change.' Japanese has two parallel ways to form the potential of a noun-する verb: the contracted ~できる and the longer ~することができる. Both are correct; the shorter form is more natural in everyday speech, while the longer form sounds more formal or explicit.