If you lost your JSON file when you first created your account and you want to make another copy, there are two ways to download it now, depending on how you manage your account.  

1. On polkadot.js web app

First, navigate to https://polkadot.js.org/apps/#/accounts

Next, click the three dots to the right of the "send" button, and select "Create a backup file for this account" as in the screenshot below.

This will only work for accounts that were created directly in polkadot.js (with type sr25519 or ed25519), not for injected or external accounts.  If you added your account through the browser extension, please scroll down to step 2.

2. In polkadot.js browser extension

To export your JSON backup file, simply click on the three dots to the right of your account and select "Export Account".  

Next you can choose a file name or leave it as is and then save the file in your chosen location.  That's it, your JSON file has been exported!

If you ever need to restore your account, you can do so with this file and your password.  For more details click here and refer to Step 3.

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