Hi 

How do you make adjustment to the cost of goods after sales? 

Hello. 
Do you want that this change of the cost affect the past sales that you have done? 
If yes, then the only way to do this is to cancel the receipt where you sold this item, change the cost of the item in the item's settings and then add the receipt again. 
You can check this help article How to cancel receipts.
You can change the cost from the item's settings as shown in the picture below: 

To enter sales with previous dates, please follow the guide below:

1. Make sure you're in online mode, then Log out of the app.
2. Change the time and date settings on your device (to enter sales from the 25th of August, change to 25/08).
3. After doing this, log in again and you can enter sales for the specified days.
4. Then log out from the app again and change back the date and time on your device.
5. Finally, log in to the app for the last time and continue enjoying Loyverse.