We can Install Android marshmallow 6.0 in motorola Moto E 1st gen by flashing Cm13 in it.As this is Unofficial Cm13 rom for Moto E 1st gen so there may be bugs in it you change check the source thread
for XDA for the bugs and stability report.
We already shared the guide of installing CM13 on Motorola Moto G 2nd gen and CM 13 for Moto G 1st gen and Cm13 for Moto E 2nd gen earlier
you can check it out Moto X play and moto X
Style/pure etc.So follow below guide of Install unofficial Cyanogenmod
13 Rom in Moto E 1st gen 2014 [Condor] .
4.Note before flashing rom do check the stability report of CM 13 from HERE.
#How to Install Cyanogenmod 13 on Moto E 1st gen [condor]
CM13 Rom for moto e 1st gen 2014 and the appropriate Gapps 6.0 package
from link given below Copy it in your in your device.
2.Now place the Cyanogenmod 13 rom and gapps 6.0 zip file in sd card.
3.Now reboot to Recovery mode and flash Android marshmallow rom in your device.
4.Then flash CM13 android marshmallow rom and gapps from recovery in it.
5.Now Clear cache and Clear Data.
6.Reboot your device.Done! you just flash cyanogenmod 13 rom in your Motorola Moto e 1st gen 2014.Enjoy Android Marshmallow 6.0.