Go to the invoice that you wish to unreconcile, there is a small "i" icon next to the payment made at the bottom. click that for more information. a tiny popup will appear with a button that reads unreconcile. 

I'm not sure about V10, but V12 allows to reconcile items belonging to the same ledger and same creditor/debtor. there would be a text ribbon on top of the invoice and some items appear in your total that can be reconciled in your invoice.

If you want to reconcile items from multiple journals like selling and buying from the same company, you will need some sort of community module/customization as odoo base doesnt allow you. you can search for a module called Account Netting by OCA