Changing the Time signature of a score


It's easier than that. Just select the entire score (Ctrl+A) and then  use
the time signature dialog (T). The time signature will be added at the start
of the score, replacing the pre-existing initial signature.

If you select a bar or range of bars then two time signatures will be added.
One at the start of the selection and another at the end. The signature at
the end will restore the old signature.

the same way, so if you learn how to insert one, you'll have cracked them
all. Just remember that clefs and instrument changes are staff object
whereas time signatures are always system objects. Key signatures can be
either staff or system items, depending on whether you check the "one staff
only" check box in the key signature dialog. (You don't need to add keys
signatures to deal with transposing instruments, Sibelius does that
automatically.)