The easiest way to explain it is just like our computers. When you get a new, blank computer and turn it on, leave it sitting for a month or so. The computer, through trying to run, will make accidents and create better and better programs the longer you leave it.
The fact that many people have programs like Adobe, Microsoft, GoogleChrome, etc. is all just convergent evolution. DNA is far more complex than computer code, but since we know computer code creates itself, so can DNA.
/s
Let's try a little experiment: DNA is said to chance in response to external stimuli changing. So what if I merely change one random bit in a complex; no; SIMPLE computer program.
What are the odds that the effect will create a functioning program?
What are the odds it'll create a BETTER functioning program??