I would like to change the folder but I don't know where can I configure Tranmission.
And if you're using Transmission from the web located at
Click on the wrench at the bottom-left of the page:
Then set the Download to: field to the desired location. This will be the default location:
And in case you have torrent(s) who's location you want to change. Then right-click on any one of the torrent(s) in the list and select Set Location...
and then set the location to wherever you want it
all the existing data belonging to that torrent will be moved to that location.