You are able to edit Task Checklist in the Web Portal > Task > + New Task > Select the Checklist from the drop down.

Each item can be edited or deleted, you also have the option to Add New Items the the check list. Once you have made the relevant changes, ensure you Save & Open to complete the check list or Save & Close to save the changes made for future use.

The new checklist will update for new tasks and will not update for existing tasks that have been created before the changes were made.