On a Records page there is new Update Multiple records form. Just select several records, select column to update and enter common value for this records and hit Update, all records will have new value in this column then
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.