Is this a program/app already installed on your 3DS? If you like to find out some of its meta information, you can check using FBI, at the Main Menu when it first loads, go to Titles and scroll down to find the app in question. This will give you Icon, Title ID, Version, Product Code, Region, and Size.
If you want to extract/decompile an app already installed on a 3DS, you will need to use GodMode9.
- In GodMode9, hover over to [A:] SYSNAND SD but do not press (A). Hold the (R) shoulder button and press (A).
- You should see some options pop up. Go to Search for titles... Scroll down the .tmd list to find your installed app.
- Press (A) > TMD file options... > Build CIA (standard). Once it is built, press (B) to go back to root.
- Go to [0:] SD CARD > gm9 > out to find your app in CIA format. Press (A) on it.
- Select CIA image options...
- Select Mount image to drive and press <A> yes. You should now be in [G:] CIA GAME IMAGE which list all the files within the mounted CIA.
- Hold the L-shoulder button and scroll down to highlight everything in yellow.
- Press A on one of the '.bin' or '.app' items you've selected. The first one/two items listed with a bunch of numbers and zeros are directories.
- You should now see on the bottom screen the option Copy to 0:/gm9/out . Select <A> yes.
- In the gm9/out folder, you should find all the extracted files from your CIA which you can now copy over to your computer to edit and rebuild.