"app not installed" Replace ROM-based app with modified APK without root I'm facing trouble installing a modded .apk if the same app is already provided in stock Rom.

I'm using Micromax canvas E481, not rooted yet. Which has saavn app already installed. I uninstalled it to install a mod version of saavn. But I'm getting error "app not installed" however same apk is working on my other devices. Same is happening for playstore.apk.

Saavn is not a system app, it's just provided in ROM by Micromax. From application, it gives option to uninstall it. I can install apk for any version of Saavn, it's just not letting install the mod version. If I modify it using lucy patcher it won't install it but it will let me install the original apk for any version bypassing the pop up from es Explorer.
INSTALL_FAILED_UPDATE_INCOMPATIBLE should tell you something. "app is already provided in stock Rom", "not rooted" and "I uninstalled it" don't match: w/o root, you cannot have uninstalled a system app (you probably have disabled it).

Same goes for Playstore: you cannot simply "update" it with a patched version. If it's an official version (signed by Google), you could update it – but then you must tell adb install you want to update: adb install -r xx.apk (note the -r, which stands for "re-install"). Not sure whether that works via adb for pre-installed apps, though (I've never tried).