To clarify..... the previous answer is not correct.
You can withdraw your application at any point before the visa is issued. Please see pg. 13 of the Sponsor's guide:
"If you change your mind about sponsoring your spouse, common-law partner, conjugal partner or dependent children, you must write a letter to CPC-M before they are issued permanent resident visas (see What Happens Next? for information on the application process).
Once permanent resident visas are issued, the promise you, and if applicable, your co-signer, made to support your family is valid for the term of your undertaking."
Fernandita, can I ask why you want to withdraw your application and apply outland? Outland is not any "harder" than inland. With both applications, you will need to submit proof of the genuineness of your relationship. What country are you originally from? With some visa offices, applying outland can take just as long, if not longer than inland. If you give us some more details of your situation, maybe we can give you better advice.