When I try change a script var value in inspector it switches to box collider field. Is it a bug?

That does seem strange. My first reaction is that you've set your script to execute in edit mode and you set the collider center y value to your the lives value in your update loop...like you're trolling us. But that couldn't be, could it?

If this is legit then it looks quite strange and bug like. Have you tried restarting Unity? Have you filed a bug report?

No, not trolling at all! And no I've nothing strange like that happening - it's still in the very early, basic stages. Literall just a var in the script which I want to set a value to. It does seem to change sometimes but I couldn't, for instance, put on a 2 digit integer because after the first or would shoot up to the other input box. I've not filed a bug report as I wasn't sure if I was doing something silly and thought I'd see if anyone here has any advice first.