ex : I want the tempo to be 180 at bar 8 , is it possible to do that with sibelius ? ( i didn't find anything related to variable tempo in the very large documentation )
Alt+ctrl+t, to get the blue insert arrow, then click over bar 8, then right-click at the blinking cursor, choose, say, the quarter note symbol, and type =180 with no spaces.
You can actually then drag it around, and have the tempo change within the measure elsewhere. If a note is selected when you alt+ctrl+t, then the cursor will blink at that location.