Set up a script to select all check boxes with this script step:
Set Field [YourTable::CheckBoxField ; ValueLIstItems ( Get (fileName) ; "ValueLIstNameofCheckboxesGoesHere" ) ]
Use this script to select all the check boxes in your field. Put a button on your layout to select all the check boxes. After clicking, the user can click individual check boxes to deselect them.
Thank you Phil!
Is the filename the name of the valuelist?
No, it is the name of your database file. That's why it's called Get ( FILEname) You could put the actual name of your database file here in quotes, but then renaming the file breaks this calculation so you want to use Get (FileName) so that you always have the correct name as that first parameter.
Get functions can be easily looked up in FileMaker Help if you see one that you do not recognize.