How to recall troops from a wrong sending command?

To recall a legion you must first have completed the research 'Recall Order' found under each of the legions research lists. (about half way down). This unlocks the option.

When the legion is marching you can then tap on if and you will have the option to recall. You will need to use a 'March recall' item to make the recall. These can be earned in rewards or purchased in the store for gems. If you don't have any then you can also just pay gems at the time to make the recall.

You can also purchase recall orders from the store when click on your gems in the upper right hand corner.