Make sure you save before you regenerate!
If you have manually removed an employee who would traditionally appear on this run (has an active or on paid leave status for at least one day of this pay period) they will reappear after you regenerate.
If you have manually removed a payroll instruction that would traditionally appear on this run (as per the schedule settings for that payroll instruction) that instruction will re-appear after you regenerate.
Any employees or payroll instructions you added to the input sheet will stay. Any data you've entered into the boxes within the input sheet will stay, although data that pulls from default values may update if the default value has been updated.
The Regenerate button refreshes all data connected to the employee profile - this includes their compensation, their banking information, and their default values. If you have made many changes to employee information since you created the input sheet, but you've also entered data into the input sheet, this is an easy way to update the employee information without losing the entered data.
If you've only changed information for a single employee, it may be easier to remove them from the input sheet and add them back in - this does not save entered data for the one employee, but does ensure no one else is adjusted. More information here.
If you've only changed information for a single payroll instruction, it may be easier to refresh the payroll instruction - this does save entered date, and refreshes default values.