Normally, a reskin is a "simple" matter of finding which texture file (there may be several) is the one you have to edit, load it into a graphics editor, do the edit, save and then submit the edit in CM. All my reskins have been like that. Obviously, you do this on a clone of the original, never on the original itself (unless you really like pain).
Having said all that, if you have not performed a reskin before it can be a mystery as to which parts of the texture file have to be repainted. The loco is a 3D object and the texture, or the "skin", is a flat 2D surface that is wrapped around it - very much like a bear skin rug. Matching up the parts of the texture file to the corresponding parts of the loco can be tricky.
The logo itself may be part of a different texture file.
Be prepared to experiment.
My experiences anyway.
Development: Trainz Plus - b119450
Testing: Trainz Plus beta - b119450
GTX1050Ti, RTX3060Ti and RTX3050Ti, Win 10 & 11