./hmy --node="https://api.s0.p.hmny.io" staking undelegate --delegator-addr one1pf75h0t4am90z8uv3y0dgunfqp4lj8wr3t5rsp --validator-addr one1pdv9lrdwl0rg5vglh4xtyrv3wjk3wsqket7zxy --amount 0 --chain-id testnet --passphrase (yourwalletpass)
This undelegation transaction will be signed using the delegator-addr. Hence, the delegator-addr should exists in the keystore.
When you decide to undelegate your tokens from a validator in Pangaea your tokens will be subjected to a cooldown period lasting 7 epochs / 70 minutes.
This means that you won't have access to your tokens or can transfer them for 70 minutes after choosing to undelegate your tokens from a validator.
You can read more about epochs in Pangaea here.