how to close activities shown in a grid ( checkbox)

Hello,

I have a Grid with all activies that i want to close.

The user will have a checkbox and all marked activites should be closed.

I created a form with Type Object 33 ( OCLG) with a grid showing all still open activities and the last column is the ODLG.Closed field which can be checked or unchecked.

How can i then use an update Button to update all checked activities and set them to close?