I see four possible issues:
1. A physical problem with the Fn key on your laptop.
2. A setting problem related to the Fn-key
3. A driver issue. Did you check on Dell.com if there is an updated driver for Windows 10?
4. A general incompatibility between Windows 10 and this old program. Then maybe http://sourceforge.net/projects/turbocforwindows-9/ helps?
Does it work correctly if you use an external keyboard?
i am very upset that i installed windows 10 about 1 month ago. But now i have a issue that when i use Turbo C++ then, turbo C++ software gives the option "Press F1 for help'', "F2 for save"and "F3 for open" but when i press these buttons then no it doesn't work. Means i mute the volume and slow down the volume and or increase the volume. And my laptop is Dell core i3 inspiron 15.I also wanna tell you that these buttons are use for mute,slow and increase the volume when i don't use turbo c++.