Memory can be expanded by increasing the number of words and keeping the number of bits per word same. This is known as Address Expansion.
A 32×8 PROM has 5 (32 = 2^5) address lines [A0-A4] and 8 output lines [D0-D7]. A 64×8 PROM must have 6 (64=2^6) address lines and 8 output lines.
A0-A4 form the five address line of 64×8 PROM and the chip select line forms the sixth address line. The image below shows that a 64×8 PROM can be made using two 32×8 PROM.