By creating a new migration which updates the table: https://laravel.com/docs/8.x/migrations#updating-tables
you would make sure you either add a nullable column or a column with a default value which all the other rows will inherit.
Make sure you make a backup as well, and that's it. But the critical part of the migration is when you create the table Schema::create is used when updating Schema::table is used. And of course you will test locally with existing data. But as long as you are updating the table, no data will be lost, that's the benefit of the migrations as well.