Prepare your workspace, tools, etc. this is important due to the challenges faced in handling SMD electronic parts. Decent light conditions, non-static environment, etc. A SMD Soldering guide link is given on step 2 and 3.

It is recommended to purchase a batch of transistors (say 5-10) for different reasons: The failure might happen again, SMD parts are very delicate in handling, because you love transistors, etc.

Note: If you cannot find a BC817 replacement say at Mouser, Digikey, etc. this transistor from KEC provided by Conrad DE is confirmed as working

Note: If you do not have any electronics soldering equipment, take a look at this starter kit or for Europe (Germany) try this set.