I would like to change all transitions in one go from 1 to 2 - how can I achieve that

I am running iMovie 10.1 with El Capitan on a Mac Book Pro.

I am able to make a global transition duration change by double clicking on a transition, changing the duration in the in the pop up information box, and then clicking on Apply to All. You said that that didn't work for you, so you might want to try the following:

First, duplicate your project and work on the duplicate so if things go awry you still have your original project intact.

I did the following on a project that has no theme, the transitions were all cross-dissolve, and the Automatic Content box in Settings was deselected.

Go to iMovie/Preferences and set the transition duration preference to what you want.

Go to Edit/Select in iMovie/Transitions.

Hit the delete button on your keyboard. All transitions should delete out. (If they don't, then,

in the pop up box that displays, click on the button "Turn off Automatic Content". )

The transitions will disappear.

Then go to Settings (upper rt. hand corner of Timeline) and select the "Automatic Content Box".

Your transitions should re-appear throughout your project with the preference duration that you have set.

They will all be cross-dissolve.

Hope this helps. At least, it worked for me.

If it works in a new project but not your current project, then you might try creating a new project with the assets that are in your

current project. Duplicate your current project and work with the duplicate. Then do Edit/Select All and then Edit/Copy. Then open a

new iMovie project and Edit/Paste the assets from your current project into the new project. See if that works.

Sometimes, for no apparent reason, a particular project will develop a glitch where some stuff doesn't work. I have found that creating a new

project will often solve the problem.

As an aside, when you double click on a transition and change the duration in the information box, don't click on Apply but go straight to Apply to All.

Good luck with this.

In iMovie preferences you can set the default transition duration to 2 or whatever you want. Preferences are in iMovie menu item.