Doc gave you the correct steps
locate the start-button sometimes its a .tga file if it is animated...( if it is animated you need to open it in Photoshop for example or your favorite painting program, check the picture , if it has 7 buttons on it for example you will need to divide the picture´s width by 7 to get the exact lenght of one button (if there is transparent pixels between them) example:Animated startbutton with 7 buttons with a total picture width of 490 and height 60... 490 width divided by 7 = 70 width and 60 height- ( single button ) the shape should be equal or it might look strange if people have set margins on the startbutton...If you completed your button Proceed with renaming the original and continue with step 2( for animated buttons ) line them all in the end your picture size should be W490/H60 again ...not more not less... (or whatever the size W/H the picture is that you try to change since that was only an example )
1. Rename the Start Button you wish to change by adding the extension .bak at the end of it's regular name. Leave it in the same folder, so you can change back in the future.
2. Drop the new Start Button (make sure it is the right type, ie .png, or whatever) into the same folder where the old start button is.
3. Reload the skin.
If you wish to change back in the future, remove the .bak extension from the original, and put it on the one you wish to change.
code values can be changed as far as i know without Skinstudio ( margins )but needs finesse and i don't know if it is legal/illegal without Skinstudio so i shall not explain it 
The whole progress is much easier if you simply buy skinstudio if you plan on private modding skins more than once or if you feel like creating your own skin one day ... porting is a different thing since you will need permission from both sides before you can even begin...
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