The same way as you'd set the type (number format mask) after writing a value to it:
$objPHPExcel->getActiveSheet()
->getStyle('A1')
->getNumberFormat()
->setFormatCode(
PHPExcel_Style_NumberFormat::FORMAT_GENERAL
);
or
$objPHPExcel->getActiveSheet()
->getStyle('A1')
->getNumberFormat()
->setFormatCode(
PHPExcel_Style_NumberFormat::FORMAT_TEXT
);
Though "Number" isn't a valid format mask.
You can find a list of pre-defined format masks in Classes/PHPExcel/Style/NumberFormat.php or set the value to any valid Excel number format masking string.