Navigate to Settings > Profiles/Policies > Schedules > Work Schedules. Those are all of the schedules you currently have built. You can create new ones as well as edit the current ones.
Once you have set up what you want to use, you will navigate to Settings > Mass Edit > Mass Edit Profiles.
Select all of the employees that you want to change to a certain new schedule and then click on Mass Edit.
A screen will open to Mass Edit Employees' Settings - find the Work Schedule Profile, select the new schedule profile you want them to have and then Apply Change.
After you have assigned the new schedules to the employee profiles using mass edit, you must then Generate the Schedules. Navigate to Team > Schedule > Basic Scheduling > Generate Schedules.
Select all of the applicable employees and Generate Schedules.
From Date: Start date of new schedule
To Date: End date of schedule or put 12/31/2099 (end of time)
Work Schedule: leave blank (it will use what you put in the employee profile)
Override Existing Schedule Entries: check the box to override any schedules previously generated
Generate