Hello Tomas, and welcome to Seven Forums.
What is the brand and model of your mouse? If we know which one you have, then someone with the same one will be able to help better.
For now, click on the
Mouse icon in the Control Panel (icons view) to see if you may have a
Settings type tab like in the screenshot below. This will vary depening on what brand you have, and if you installed the mouse's
drivers/software.

I have a Logitech mouse, so I get this when I click on my
SetPoint Settings tab. From here, I can change what the middle button function will be. This is brand specific though.

Hope this helps for now,
Shawn