Once a prescription has been created in a patient's file, if changes need to be made you can go back to the prescription screen and select the grey pencil next to the one you want to edit.

From here you can remove anything by selecting the red trash can next to the item if necessary

You can also add additional items, the same you would if you were creating a new prescription. Just follow the instructions for creating a manual prescription for any other changes. and be sure to select save once complete.