1. Go to Settings -> Messages -> Encryption.
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2. Click on your Personal key for which you would like to change the expiration date of.
3. Click on Edit next to the current Expiration date.
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4. Enter your passphrase for that OpenPGP keypair and click on OK.
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5. Select the new Expiration date OR CheckThis key does not expire box.
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- If you already have published your OpenPGP public key on Public key servers, then Check Publish on public key server box to update your OpenPGP public key expiration date on public key servers.
6. Click on Save.