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In the user settings for a specific SQL Developer version (for Windows, for example, C:\Users\<yourID>\AppData\Roaming\SQL Developer\system<SQLDevVersion>) search the folders below for
connections.xml (for SQL Developer 18.2 and below)
connections.json (from SQL Developer 18.3 and above)
Either edit the connections found there or rename / delete the file if you want to import from scratch.