I haven't done it on a Tytn but I have done it on an O2 XDA IIi, and I found it quite straightforward to be honest, but I am a qualified electrical engineer. It is not changing the screen that is difficult, that is just a matter of disconnecting ribbon cables and slotting in the screen, it is the dismantling of the device that is more troublesome especially if there is a combination of screws and snap together joints that can easily be damaged, but if you take your time it is quite straightforward.
As you have probably seen the Tytn is not that difficult to dismantle and if you feel confident about doing it then go for it. Ebay is the best place just check out the dealers reputation or try
HERE they are a well used company.
Is it a screen fault you have though?!!
There is more info on the problem
HERE and correct part numbers
HERE 
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O2 Ignito (HTC Diamond), E-TEN Glofiish V900, O2 Orbit 2 (Polaris), HTC Trinity, Vodafone v1615 (Kaiser), HTC Tytn (Hermes) + Others!
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