Update a record of DB if it exists after importing a Csv file in Laravel?

You should implement the WithUpserts interface. Also you need to specify which field should be unique so if it exists, it will update the record, else it will create new one.

Assuming your unique column is email, change your code to the following:

class UsersImport implements ToModel, WithCustomCsvSettings, WithHeadingRow, WithUpserts
{
    /**
     * @param array $row
     *
     * @return \Illuminate\Database\Eloquent\Model|null
     */
    public function model(array $row)
    {
        return new User([
            'firstname' => $row['firstname'],
            'lastname' => $row['lastname'],
            'email' => $row['email'],
            'password' => $row['pass'],
            'created_at' => $row['registered'],
        ]);
    }

    public function getCsvSettings(): array
    {
        return [
            'delimiter' => ","
        ];
    }

    public function uniqueBy()
    {
        return 'email';
    }
}