How do I upgrade my Dell XCD35 from 2.2 Froyo to 2.3 Gingerbread?
Root it.
Install Clockworkmod Recovery.


Install ROM Manager.
Run it and select your device.
Choose the "Flash Clockwork recovery" (or similar) option.

Install a custom ROM.


Note that doing this voids your warranty and may brick your device.  I highly recommend reading through the XDA forum (the last link above) so you get a good grasp of everything involved here.  In particular, a lot of the ROMs are for "GEN2 devices".  I don't know know if that's a hardware revision or a firmware version, or if there are any relevant differences between the Dell XCD35 and the ZTE Blade that you would need to account for before installing a Blade ROM.  Edit: See http://wiki.modaco.com/index.php/ZTE_Blade#What_different_Blade_versions_are_out_there.3F for some useful info, the rest of that wiki is useful too.