When you start a new score, you can choose from a long list in the set-up of the different speeds.

But I don't know how to change it during the piece.


There's blood in the water
Sinking, always sinking

click on the note where you want the legato to start then crtl + e and then right click next to the curser and a menu will come up. From there click legato and you're set.
It doesn't seem to work. I click legato, and it shows up on a little box, then I press somewhere else on the score and a bunch of random letters goes where legato was. And I don't notice a sound difference.
Sounds like you've either selected something you shouldn't have or you're sibelius has some errors. Also it will only work if you have the appropriate sounds selected? Are you using the Garitan/Sibelius sounds? If you're using an external library you most likely wont be able to affect the performance through notation aside from basic dynamics. Good luck, sorry I can't help more.