On your shopping list screen, swipe left on the shopping list row that you want to edit as shown below and tap on the edit icon.
You could set retailer timings in a couple of different ways, one is to automatically populate a retailer's store timings (if one is available) by linking a shopping list to a retailer location. Second way is to manually enter retailer store timings.
To link a shopping list to a retailer location, tap on the "Tap to pick a location" row under "RETAILER INFO & PROXIMITY ALERT" section. If you are prompted to provide location access, select "Yes/Allow" so that Grocery King can locate your location to find retailers nearby. Grocery King will not fetch retailers from the shopping list name you provided. If your shopping list name is not a retailer name then you could enter the retailer's name and search again. Select the matching retailer that is closest to you.
Grocery King will then fetch retailer timings if available and populate them on the Edit screen automatically. If Grocery King cannot find the retailer timings then you will have to enter each one manually by going to each day the retailer is open.