I don't have an Eris, but you can try this:
http://forum.ppcgeeks.com/showthread.php?t=120749
Scroll to the bottom of the 1st post and there's a MegaUpload zip file that you need to download (which is the 2.1 leaked update), then follow the instructions at the bottom of that post.
No promises, but that's said to be the same version as the OTA update.