Hello Antony,
in the Update Data store event you can filter which records to update using the “Which Reords” to update.
Filter by your is_selected (enter the current value) and enter the new value you want to add there.
Example - all the records witch have a task value == 5 will be updated to 0.
Add an extra property, such as ‘allitems’ to the data store and put a 1 in it for every record when the record is added. That way you can identify all records that have a value of 1 for allitems