where is hikashop_previous_product

-- HikaShop version -- : 2.6.3
-- Joomla version -- : 3.4.5
-- PHP version -- : 5.5
-- Error-message(debug-mod must be tuned on) -- : none

hi acn you tell me where is hikashop_previous_product and hikashop_next_product. i would like to add text to it. something like go back and next page

thanks