If you would like to set schedule for all devices in the store at once, please follow the below steps for assigning a schedule to all devices in a store (added under one group) :
1. Navigate to the Publish module and click on + Publish :
2. Select the Schedule option :
3. You will see all screens listed. Now, click on the filter icon and you will see a popup having groups list. Select the group (Eg. Store 1) that you want to assign the schedule & apply the filter.:
4. It will list out the screens belonging to the group selected. After this you can use select all option to select all the screens associated with that group and then proceed further to create the schedule.

If the schedule is already created, you can use the edit option available in the schedules list & follow the same steps from 3rd point.