You can edit Communications that have been previously saved in the system. Note - you can only edit notes made by yourself and not other Users.
If the communication had been set up as a diary and/or to-do list entry in the Communication Window, these entries will automatically be updated when the Communication note is changed. Similarly, if a diary or to-do list entry is edited, the communication note in the record will also be updated.
You can only delete communications that you have made yourself, you cannot delete other user's. Communications that have been sent to a company contact as an email cannot be deleted at all, even if it is an email you have sent yourself. The email communication will stay as a permanent record of what has been sent.