Well, the update to Android Lollipop 5.1 is available for download for Moto E 1st Gen devices. You should receive a notification for the update on your device. If not, check for the update manually.
If you have received the notification,
1. Select Yes, Im in
2. Software will download on your phone. Once it is completed, select Install Now.
3. Your phone will restart after the successful installation.
4. Done - your phone is updated to the latest version.
If you received no notification,
1. Go to Settings.
2. Select About Phone.
3. Select System Updates. You will be notified if the update is available.
4. Follow the instructions as above.
Please note that while updating your phone, you need to have at least 50% or more charge. Make an effort to stay in a better coverage area. Updating your phone using WiFi would be much safer.
Live....and Let Live!