The process to edit orders, invoices, credit memos, and shipments is the exact same:
1. Go to Sales → select what you would like to edit
2. Select the item(s) that you would like to edit
3. Click Edit in the tope left box marked "Order #XXXXXXX"
4. You can edit the following:
- Order Number
- Order Date
- Order Status
- Order State
- Store Purchased From
- Invoice Number
- Invoice Date
- Invoice Status
- Credit Memo Number
- Credit Memo Date
- Credit Memo Status
- Shipment Number
- Shipment Date
5. Click Update in the bottom right corner of this block
6. You can also choose to notify the customer by clicking the "Notify Customer" box before clicking Update