Not sure which password that you are referring to, since the Time Capsule has up to 4 different passwords.
If you are asking about the password to Encrypt your Time Machine backups, this cannot be changed from the original password that you used to Encrypt the backups.
If you are asking about how to change the Disk Password for the Time Capsule......
Open AirPort Utility on your Mac (Finder > Applications > Utilities > AirPort Utility)
Click on the picture of the Time Caspule
Click Edit in the smaller window that appears
Click the Disks tab at the top of the next window
Click the Secure Shared Disks selection box
Choose With a Disk Password
Change/Edit the existing disk password
Change/Edit the password to verify
If you are asking about how to change the Base Station password for the Time Capsule......which is the default password used to access Time Machine backups......unless you chose to use a separate Disk Password.....
Open AirPort Utility on your Mac (Finder > Applications > Utilities > AirPort Utility)
Click on the picture of the Time Caspule
Click Edit in the smaller window that appears
Click the Base Station tab at the top of the next window
Change/Edit the Base Station Password
Change/Edit the Base Station Password again to verify
If you are asking about how to change the Wireless password for the Time Capsule......
Open AirPort Utility on your Mac (Finder > Applications > Utilities > AirPort Utility)
Click on the picture of the Time Caspule
Click Edit in the smaller window that appears
Click the Wireless tab at the top of the next window
Change/Edit the Wireless Network Password
Change/Edit the password again to verify