In this case, you're either going to have to use count (starting from 0), or a findElement
I think you can probably use
driver.switchTo().frame(1);
Or, and I think this may be the better solution
driver.switchTo().frame(findElement(By.css("form>iframe")));
especially if the form has a name or id.
i.e.
If you are going to css select based on form name X, then the css selector will look like this, where X is the name of the form. You can directly substitute the name for the form where the X is.
"form.X>iframe"
by form ID would look like this:
"form#formID>iframe"
where formID is the ID of the form.