In Attack on Titan, the blades of the 3DMG weapon are swapped many times. What button/lever do they use ex; thumb button, trigger 1, trigger 2?
I think I found the way of changing the blades. But I didn't find the use of it while changing the blade. So I can't say that this is true.
The description about 3DMG or it working functions has been disclosed in the anime episode as image in the interval part(mid episode transition part). But the working mechanism of the handle which is used to control almost all the part 3DMG is stated as black box and hasn't been disclosed.
For changing the blade, it is the trigger like thing or Thumb button on the left side of pic above which is similar to the one in the rear side of the revolver pistol which is pulled outside to fire.
As shown in gif below, trigger 1 and trigger 2 are for shooting the arrow or hook. And the lever is used for the similar purpose, like releasing the gas to change direction. Combination of Lever and triggers are used for agility.
One more thing that made me think is, In episode 7, around 15 minute while Mikasa return the broken blade, nothing is shown. She simply moving the handle and detach the blade. If that was the case then fighting with titan would have detached the blade many times.
Here is the YouTube video where she removes the blade.