How can I back out of a signed an agreement to sell my house? 

You should also look at the purchase agreement.  If you used the standard California Association of Realtors form, it has a liquidated damages provision.  If both you and the buyer checked or intitialed that provision, even if the buyer fully performs you can breach the contract for the amount set forth in the liquidated damages clause, generally 3% of the purchase price, sometimes less.  I'd have to look it up but I'm pretty sure it cannot be more.

Disclaimer:  The materials provided below are informational and should not be relied upon as legal advice.

Genenerally, an executed contract is binding on both sides; any party that "backs out" has breached the contract and will be liable for damages to the other party.  There are some exceptions, such as where the contract is not validly formed (e.g., no consideration given), or if the buyer is unable to satisfy conditions precedent to the purchase, or if both parties agree to terminate the contract, in which case you will be relived from performing under the contract.  You should consult your own attorney to protect your legal rights. 

You probably can't, unless the buyer fails to meet or comply with some condition of the contract. For example, if they have a financing (mortgage) contingency and either fail to get a mortgage or fail to get one by the date indicated, you could pull out. However, it buyer meets all their obligations, you are reciprocally forced to honor your obligations, too--a contract is a contract. If you simply refuse to sell, buyer may be able to force you to sell by taking you to court; even if, for some reason, the court doesn't compel a sale, buyer will almost certainly be able to win substantial monetary damage. You should go ahead as if you will have to sell but in the meantime, watch buyer's performance like a hawk and swoop down on any noncompliance or failure.