Obviously, yes, you can withdraw.
How long ago did you accept and when are you supposed to start?
Is it something relatively fungible (e.g., are you going to be an associate at a large law/consulting firm where it will take them all of 30 seconds to call the next candidate to replace you) or is the position somewhat unique (i.e., it will be hard to find someone else)?
Do they still have time to find a candidate before you were supposed to start?
If so, be apologetic, but you can rescind your acceptance. Basically you're just quitting early.

But recognize that they went to effort and expense to select you. How much your rescission will inconvenience them is an open question, and that measures the degree to which you might be burning bridges.
What kind of industry is it? If it's small, then others might hear about you as someone vents that they now have to start their search over again.
All things to consider.