You wanted to manually update your Droid Eris to Android 2.1, and so you shall. Here's what you need to do:
- Download this file. (Thanks, AndIRC!)
- Copy it onto your SD card. Be sure not to put it in any folders.
- Turn you phone off. Now hold down the power and volume-down buttons to enter the bootloader. Follow the instructions to update to Android 2.1
Easy as pie! (Update: HTC reminds us that updating via this method will wipe your phone, and that waiting for the OTA update will not. Good to know.