You can usually do this if you still have the Mac that you used to originally configure the device.
See this User Tip from forum expert Tesserax:
If no luck, you can perform a Soft Reset on the router without changing the network settings at all. A Soft Reset will give you 5 minutes of free access to go in and simply change the passwords without affecting anything else. Might be a good idea to write those new passwords down if you go this route.