1. Open the desired texture.

2. Repaint.

3. Save in the same format without renaming the file.
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Of course, the "problem" lies in item 1.  "Open the desired texture".

Typically, the textures are not "standard format" bitmaps - you won't be able to simply open the program in your paint program, but rather you'll need a converter.

A very good repainting tutorial is at:
http://www.calclassic.com/repaint.htm

"Jargon" is unavoidable - the different texture formats useable, the converters themselves (DXTBMP, imagetool), etc.

For a flightsim specific paint program, you may want to consider Abacus' FS Repaint - in which you can view the 3D model and see your changes as you make them.  check their website: http://www.abacuspub.com