If you edit any of the steps in an active Sequence, the changes will only be applied to newly enrolled Contacts. Any changes you make here will not affect those already enrolled in the Sequence, as you'll see in an alert that will appear at the top of your Sequence.
If you would like actively enrolled contacts to receive your updated Sequence, you will need to first unenroll them from the Sequence and then re-enroll. You will have the ability to re-enroll the contacts in any step of the Sequence.
When enrolling a contact, select the dropdown labeled Initial touch and select the whichever email you'd like to the contact to begin the Sequence with.