How to rekey your account
Set / Add a regular key pair to your XRPL account
The XRP Ledger allows an account to authorize a secondary key pair, called a regular key pair, to sign transactions on an account. It is often used to protect your account by allowing you to sign transactions for your account instead of using the master key.
A good example of how this works would be to look at the recommended configuration of a pair of Xumm (Tangem) cards.
In this case, you have a Primary account on one card and you have a Signing account on another card. When you run the Tangem Backup xApp, it creates a "link" between the two accounts which is called a regular key pair. It connects the Primary account to the Signing account which allows you to sign transactions on the Primary account using the Signing account.
The following steps explain how to manually configure a regular key pair between two XRP Ledger accounts.
The Primary account must be imported into Xumm with Full access. (You must be able to sign transactions on this account.)
This is the account that will have your XRP, your Trust Lines, Escrows, DEX transactions, etc.
If you are new to Xumm and don't have an XRP Ledger account yet, you can create one by following the instructions in this article:
The Signing account must be imported into Xumm with Full access. (You must be able to sign transactions on this account.)
This account does not need to contain XRP and does not even need to be activated. As long as you have full access to the account, it can be used to sign transactions on the Primary account.
If you don't have signing account yet, you can create one by following the instructions in this article:
Once you have completed the following steps, you will be able to sign transactions on your Primary account using both your Primary account and your Signing account.
1) Launch Xumm and switch accounts to your Signing account.
2) Press the Request button then press the Copy button.
3) Now, switch accounts to your Primary account in Xumm and press here to go to XRP Ledger Services.
4) Press the 
button then press Open in Xumm App and sign in.
5) On the XRP Ledger Services page, select Set Regular Key
6) In the field below paste the r-address for the Signing account, then press the Send Set Regular Key to Xumm button.
7) Sign the transaction in Xumm and you're all done!
Now when you look at your Signing account, notice how Xumm now reports "Regular key for Primary Acct".
If you configured the regular key so you would have a second signing account (E.g. as a "backup" account) you are all set to go. You should be able to sign transactions on your Primary Account wih either your Signing account or your Primary account.
...but if you configured the regular key because your Primary account has been compromised, you are only half way there. The next step is to disable the master key for your Primary account.
This article explains how to proceed: