I have created form with axMapControl and comboBox with values of possible scales (e.g. 1:1 000 000, 1: 500 000, etc).
How can I change current map scale programmaticaly by using value from selectedItem taken from Combobox?
You can set
IMap.MapScale to set the current map scale.
I had a problem in obtaining map scales and I solved it in this answer to How to get a list of map scales programmatically in ArcMap?.