Currently it is unknown if the checklist can be manipulated in Docs. Clark Lind suggests to replace a list item with a copy of one of them (which is in expected state – check/uncheck)

A nice example of the Google Apps Script developer community helping each other out. In this case Alexander Ivanov was looking for a way to interact with the new Google Docs checklist feature. A direct way to interact with these isn’t possible but some clever coding from Alex demonstrates what is currently possible.

Source: Uncheck a checklist