To edit details for contacts which have been added within Messenger v5 and exist only in the Messenger v5 address Book, simply click on the contact name wherever you see it underlined. You will then see their contact record, along with some historic message information, where you will be able to edit or add numbers as required.
To edit details for contacts from your source database, you must amend the information in your source database, not within Messenger v5.
There is a minor exception to the rule about editing data only at the source. For example, if a parent has informed you that morning that they have changed their number and then you send a message later in the day, but have not yet updated the MIS (or the overnight sync has not taken place since you amended the MIS), you can amend the contact record in Messenger v5 so that the message you send is received on their new phone number. It is essential however that the number is also subsequently updated in the MIS as the next time the data is updated from your MIS, any changes in Messenger v5 will be overwritten. If this is the case, you can temporarily edit the data by clicking the contact name within Messenger v5.