Open the email you want to forward by tapping on it.
At the bottom of the screen, tap
to forward the email.
Edit the Subject if needed, add or modify recipients in the To and cc/bcc fields, modify the text as needed and then tap on Send. See Send and manage email for full details.
Tip: Often when forwarding email, you only want to focus on one aspect of the original email. In that case it's a good idea to modify the Subject field to concisely describe what you are focusing on and to add some explanatory text to the body of the email.