No, you use the setTime method. Lets say you want to set it to 11:30, then you'd do something like this...
//setTime(byte sec, byte minute, byte hour)
rtc.setTime(00, 30, 11)
Of course you'd need to work it into your code so your buttons increase/decrease each variable but that is another subject and if you aren't sure how to work with buttons then you might want to start back at square one.
There is more info on the library here: Arduino Playground - Rtc_Pcf8563 RTC Library