Hi,
Save your "original" string  or text whatever you use (textmeshpro, text, etc.) you can use "getTextMeshPro Text" to save your text to a string , then make sure your UI text is empty and now use a loop  with "Get String Left" or Right if your are using Japanese or Arabic format 

 using your saved string with the "Char Count" increasing each loop and store result string and display this one into your empty Ui text zone and don't forget to use "wait" to get a pause between each display if not you'll get your text shown as whole in 0.001 sec...