You can easily find your FTP details by opening your cPanel and clicking on FTP Accounts:

If you created an FTP account before (it can be done on the same page above), in the FTP Accounts section you will see presented with these buttons:

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Button to change FTP account password
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Button to change FTP account quota
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Button for deleting your FTP account completely
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Button for checking your FTP details
Your FTP details should look similar to these:

NOTES
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If your domain is not pointing to Hostinger by nameservers, the FTP server address will not work as a host for an FTP connection. Replace it with your account’s IP address
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FTP is not available for websites created with Hostinger Website Builder