We have just started using Magento but cannot seem to amend an order without it cancelling completely. Can anyone help with this?
No you can't amend magento order without cancelling, But you can use any supported extension to do that.
Here is one :
It should be helpful in your case.
I'm not sure if this will solve your problem but you can edit an order and that will cancel the original one and then will create (automatically) the same order number with the suffix -1, -2, etc.
That's an out-of-the-box feature.