The thing you should remember is that Hebrew and other Semitic languages don't bother to write down the entire word, unless it's vitally important (like the name of god). They just write the consonants. In the case of
Rebecca, this would comfortably give you R-B-K, and you could fill in the vowels yourself.
In English we don't find the -bk- combination comfortable, so English people felt that sticking another -e- in to separate them was an improvement. So
Rivka becomes
Rebekah, or
Rebecca. (Same thing happened with
Yahweh and
Jehovah; the Y -> J is also pretty usual, and
Jehovah has a more English sound!)
As for the -V- turning into a -B-, this is pretty normal. In Hebrew they spell
Deborah with a V - Devora - and this is just one more example.