The mid-range LG smartphone Optimus L90 was available in the US exclusively on T-Mobile, it’s sold with Android 4.4 KitKat and there’s no indication that it will get an upgrade to Android 5.0 Lollipop. This smartphone only got one software update in November 2014 with D41510e software number, most likely because it’s not a flagship smartphone just like the LG G3 which frequently get a software upgrade.
Previously, the root has been obtained on the T-Mobile LG Optimus L90 (D415) through the well-known one-click app called towelroot. In those days, the phone still running software version D41510c. But since it’s updated to D41510e, the towelroot app refuses to work again, most likely the vulnerability in the 10c software, which is employed by towelroot to gain superuser access, now has been patched.
Unfortunately, until now there is no safe way to root a D41510e software on T-Mobile LG Optimus L90 (D415). If you already updated or just got this phone and it’s preloaded with 10e software that’s mean you’re out of luck.
However, there is one solution that you should consider if you want to get root, by downgrading the software to the initial software version 10c, to then gain root with towelroot. Let alone the new 10e software does not provide any significant changes when compared to the initial version.
Once rooted you can do some fun things like customizing the UI with xposed framework, unlocking the phone’s bootloader, or even installing an aftermarket firmware like CyanogenMod 11 (CM11). But if you are fine with the stock unrooted phone, then updating to 10e is a smart choice.