Text Dictation Command for Delete or Backspace on mobile devices

• If dictation isn’t sure what word it heard, you'll see a blue line under the transcribed word so you can check it for accuracy. To make corrections, tap the underlined word, then select the correct spelling. To replace a word, double-tap it, tap , then say it again, and tap . 
• To replace a section, select what you want to replace, then tap  and speak again. To add more text, tap where you want to add new text, tap , then speak again.

https://support.apple.com/en-us/HT208343