I have an 07 i30 slx and would assume that the wing mirrors would be the same still. If you are removing just the white casing on the outside, I found it easier when the wing mirror is removed from the vehicle.
Firstly remove the glass from the wing mirror which will require some leverage under the lips using a screwdriver. Look at this picture and you will see four lips which grip the central motorised plate that adjusts the mirror angle.
It will feel like it will break, but it requires quite a lot of pressure to remove the mirror. After the mirror is off remove the cables that provide power to the heating element in the glass [take a picture or mark the positions of where the cables go]
After that, there are four screws that you need to remove that is holding the casing wing mirror casing.
After that it requires alot of fiddling to take the casing off, but take it slowly to remove the casing as you need to put the new casing back in the same way.
Tada!
Otherwise if it ends up not going on, as happened to me with one of my assemblies, I found that it is easier to remove the whole wing mirror from the body and then slip the casing on.
For that, remove the tweeter assembly [put the window all the way down and just pull the black triangular assembly with the tweeter straight out]
Once you remove that assembly there will be three bolts, remove the connector and then remove the nuts [when screwing the nut back in do not overtighten or you will sheer the bolt]
The wing mirror will just slide out then after and it should be easier to slip the casing on.
Hope that helps and hope that it is the same as the 07 hatch
