the 'false' at the end means that if the file already exists it won't be overwritten. Set this this to true and it will. Bear in mind though that this will overwrite the file and not merge it so any data in existing will be lost. I.e. any records added since install.
Actually the last parameter is CreateIfNecessary. If you set it to false and the database file doesn't exist you will get an error. It never overwrites an existing database.
You can modify CopyDBFromAssets sub and change its behavior to copy the file even if the target file exists.