Backup incremental

I apologize for my English.
I use acronis true in 2011/12.

For the first time I'm doing a backup from a
hard disk (F) to another external hard drive.

I believe that this backup is started as "incremental".

Now, for example, the folders contained in
hard disk (F) n. 100.
After two days
change the contents of a folder on the hard disk (F) - for example upload a file -.
Now the folders are still no. 100, but only one has been changed.

I wish to make the backup again:
- As I do to update only the modified folder
   without ACRONIS to redo the backup of all 100?
- If I put the check mark on the disc (F) I will again
   the entire backup of all 100 folders.

Thank you for your attention.
Greetings.