We have the same engine/trans as the Corolla ECO, see if there's any write ups on them.
The Corolla Eco only comes with a CVT, not a 6MT. I haven't changed my gear oil on my iM but every other Toyota manual gear box I have is similar. If you look from below the car you should se a drain bolt and a few inches up on the side a fill bolt. I believe they have been a 24mm socket so I'm guessing that the iM 6MT is the same. Just make sure that you can loosen the fill bolt before draining it!