You actually want the shift to happen quickly, so you roll off the throttle while engaging the clutch, hit the shift lever, then roll on the throttle as you disengage the clutch. The engine won’t go all the way down to idle while you’re doing this. If it’s all done quickly and smoothly, you almost won’t hear the engine speed change at all until you disengage the clutch and it revs down to match the new gear.
Don’t worry too much about the rev range now — you’ll figure it out in time. But if this is a NEW 250r, don’t you also have a manual with it? I think the answer to your question might be inside there.