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The best explained way of doing it is the one I found at blogger Phydeaux3’s blog. If you’re unsure whether you want to risk it, you can either not do it, or back your design up before you go mucking about with changing it. Frankly, you’ll want to back up your design anyway, so go ahead and do that. That way, if you muck it up too badly, you’ll just need to upload the backup, and you’re back on your feet.
Find the instructions here.