If you need to change one of your Ethereum account passwords using Geth you'll find the account update command will work.
If you are running your node on the testnet make sure to pass in the --testnet flag.
Here is a sample session:
$ geth --testnet account list
Account #0: {634b8e79b0155e5ac1303c2fe4b1d30f2fc6b930} keystore:///Users/brad/Library/Ethereum/testnet/keystore/UTC--2017-03-13T18-48-40.744428053Z--634b8e79b0155e5ac1303c2fe4b1d30f2fc6b930
$ geth --testnet account update 634b8e79b0155e5ac1303c2fe4b1d30f2fc6b930
Unlocking account 634b8e79b0155e5ac1303c2fe4b1d30f2fc6b930 | Attempt 1/3
Passphrase:
INFO [08-22|11:40:45] Unlocked account address=0x634b8e79b0155e5ac1303c2fe4b1d30f2fc6b930
Please give a new password. Do not forget this password.
Passphrase:
Repeat passphrase: