Is there a way to install apps on iPhone which I have in my dropbox folder?
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Put them into iTunes (drag and drop into the iTunes window) on your computer, then sync your iPhone.
I noticed you included the jailbreak tag; so you might be calling .deb packages apps. In that case; you need to use SSH to drop them into Cydia's auto-install folder, then reboot your phone twice. (wait a couple minutes after the first reboot before the second)