Generally, yes. You can withdraw your offer at any point up until they accept--by signing your offer. Check with your Realtor for more information.
One additional point: Whether or not you're preapproved doesn't make any difference. What matters is whether the sellers have signed your offer yet.
Note: If they've already signed, you may have some contingencies in your offer that could allow you to terminate the contract. For example, you might have a home inspection contingency. And if the home inspection comes back unsatisfactory, that could be an "out." Or if you're buying a condo or a property in an HOA, you're generally entitled to review the documents--often providing you a several day window. If the documents are unsatisfactory, that also can be an "out." So even if you waived the financing contingency (which you shouldn't have), there may be other ways to terminate the contract.
So, again, check with your Realtor.
Hope that helps.