First you need to select one or several records using checkboxes on the left.

Open "Action" menu and select "Update multiple records".

Then you'll need to choose a column you wish to update for these records and enter a value for ALL of these records.

 

Please, remember that there is no undo. So, please, be careful while updating your database.

Please, use Export or backup your project or form, what is even better before performing any mass operations with your data.