To find and replace a word using the mobile version of Google Doc is slightly different than the desktop version. On the mobile version, you have tap on the top right corner(three tiny vertical dots) of the google doc that you want to edit. You will see the option "Find and replace'. You will see the 'Find' space on top of the Google Doc. Write the word that you want to replace. After typing the word, tap on the 'search' button from your on-screen keyboard. You will not see the replace option yet. To replace, you will have to tap on the top right corner of the Google Doc again ( indicated by three vertical dots). You will see there are two options available. The first option is "Replace" which can be used to replace just one word and the second option is "Replace all", which can be used to replace all the repetitive words.