How can I change the angle of an object like at 19:44 of https://youtu.be/mu9J0D6ebVw?t=1167 when he adds wingtips?
I saw the tutorial and there is nothing fancy about it. You have to just rotate the edge. Here is what you need to do:
Select the edge loop. Set the pivot type to a 3D Cursor.
Press Shift+S-->Snap-->Cursor to Selected and set the cursor as pictured below.
In this case the edge is not regular. You may fix it by pressing S, then X (or Y or Z, depending on your situation) and then 0.
Now you have the even edge. Rotate it using R and moving the mouse from right to left (or input the degree value).
Extrude the edge (E). And this is it!