This is a beta version, how can somebody expect that all optional incidentals are already implemented.
Try if one of these tools[superuser.com] is helping you.

It's not incidental. It disallows me from playing the game. It takes two minutes of coding to make the Y axis into a -Y to invert it. Pretty much every early access game has inverted mouse when it is publicly unveiled so that all PC gamers can try and test it.

Inverted Mouse Option is coming soon in the following updates. We apologize for the inconvenience this has caused you.

We're preparing an update for today with some first fixes. Invert Mouse option should be included in there. It's going to be up in a few hours as creating the build and uploading takes some time. Thanks for your patience and I hope this helps.

And I moved your topic to the bug forums as I think it could be classified as that :) So we have all those things in one place. Thanks!

So happy to hear this! I just bought the game and man was it hard with my mouse being backwards! Too many years playing like a pilot... Thanks for the update/fix!

As you've probably seen already, the option is now implemented. Have fun.

Invert mouse becoming "off" (empty checkbox) when you open Controls menu for next times after you did Invert. This is not so bad itself, but real mouse invert in game becomes not inverted too after that. Please check this for youselves and please fix it. Also when you open Controls menu for first time in play session - game shows you joystick layout (but i am on kb/mouse already). Also Hints shows only default keys config/layout. So feature is useless for whom who rebinded keys. Small bugs, but they add some nagative view. Can all that be fixed before release? In other aspects i still saw - great game! Thank you for such awesome work and sorry for my bad English:)