The election is made on the initial tax return filed. If this is the first year filing, you can decide whichever method to use.
If a tax return has already been filed, then you must follow whichever was checked off. It can be changed, by filing Form 3115**.
**Do note that this is a lengthy procedure and not something you'll want to do without good reason (as the IRS will require you substantiate the reasoning behind the change of accounting method).