I want to install new winter tires for my bike, it's current wheels are 26 x 2.0 and the ones i want are 26 x 1.9, does it matter, can i still install it?
Yes, you can...Size does matter, but not in your situation..
No, you can go down as small as the rim but no smaller.
Any rim will fit a range of tire widths. If your bike came with 2.0" wide tires, it can certainly take 1.9" tires too. Here is Sheldon Brown's (RIP) chart showing compatibility. For the metric-challenged, 1 inch is 25.4mm.