There are two solutions:
You can allow all item prices to be editable at checkout. To enable this feature, go to Settings > Store > Selling > Cash Register Mode. Then, toggle "Use Imonggo like a cash register" and click Save at the top-right side of the page.
2. You can allow only selected items to have prices that can be editable during checkout. To enable this feature, go to the Stockroom, click the item you want to edit, click "Show Advanced Settings" and toggle "Allow price change at Store". Click Save.