Get <a href='http://www.swissquake.ch/chumbalum-soft/hlmv/index.html' target='_blank'>Half-Life Model Viewer [HLMV]</a>. Install it. Open it. Hit File>Load Model... Once the model is loaded, hit the Texture tab and choose the texture you want to edit from the drop-down menu. Then hit Export to save that texture outside of the model. Edit the texture using your favourite graphical program [I suggest <a href='http://www.telefragged.com/wally/' target='_blank'>Wally</a> because it's free and saves you the trouble of asking for warezed copies of Photoshop or whatnot that you're just not supposed to do here--don't get yourself banned from the forums for a reskin]. Go back to HLMV's Texture tab, select the texture you want editted from the drop-down list, hit the Import button, look at the model in the Render tab, edit the texture more if you want to, re-Import the editted texture if you want, then hit the Save Model button in HLMV's Texture tab to save your changes.
To make it easier to open MDL files with HLMV, you can edit the file type options in your Explorer view options.
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To make it easier to open MDL files with HLMV, you can edit the file type options in your Explorer view options.