There are two solutions:
You can allow all item prices to be editable at checkout. To enable this feature, go to Settings > Manage Account/Branches > Store Settings. Then check "Enable cash register mode".
You can allow only selected items to have prices that can be editable during checkout. To enable this feature, go to the Stockroom, click Edit Item > Advanced Features, and check "Allow price change at Store". This feature is only available for premium users.
Note: Option 2 is only available in premium.