How do I reverse, freeze or undo a transaction on the XRP Ledger?
The XRP Ledger is a decentralized blockchain. Part of that means, that no single person, entity, group or company has control over it. Once data has been saved on the XRP Ledger, it can not be modified or altered by anyone. All transactions on the XRPL are final and permanent.
Why can't Xumm do something to reverse my transaction?
The XRP Ledger does not have any administrative functions built into it, so there is no way for Xumm or anyone else to modify or change a transaction.
Xumm is an application that allows you to interact with the XRP Ledger and helps you manage your XRP Ledger account. It does not have administrative privileges on the XRPL.
...but Xumm runs a validator. Why can't you just undo my transaction in the next block?
While Xumm is one of the many companies that runs a validator on the XRPL, that does not entitle us to any special privileges or network access.
Per the Xrpl.org website:
"Consensus is the most important property of any decentralized payment system. In traditional centralized payment systems, one authoritative administrator gets the final say in how and when payments occur.
Decentralized systems, by definition, don't have an administrator to do that. Instead, decentralized systems like the XRP Ledger define a set of rules all participants follow, so every participant can agree on the exact same series of events and their outcome at any point in time. We call this set of rules a "consensus protocol."
So what are my options?
It depends on the circumstances regarding your transaction.
If you sent funds to a crypto exchange and they did not arrive.Contact your crypto exchange and explain the details of the transaction. Most exchanges have a process that you will need to go through to redirect your funds to the correct location.
If you accidently sent your funds to the wrong r-address
Contact the destination address and see if they will send the funds back. Do this by sending a small payment (.000001 XRP) to the account with a polite message in the memo field asking them to return the funds.
If your account was compromised and your funds were stolen.
Contact your local police. They should conduct an investigation into the matter.
If you lost the account secret for your account or your account is in "Read only" mode.
Try to locate your account secret (secret numbers, family seed, mnemonic) for your account. (The only way to access an account is if you have the account secret.) If you still have signing access to your account on your phone, (or perhaps on an old phone), you can create a new XRP Ledger account using Xumm (Settings -> Accounts -> + Add account -> Create a new account), then move your funds over to your new account using the Send feature on the main page. By creating a new account, you will get a new set of secret numbers which you can use to access your account on your phone.