I currently have following code under my "Game1.cs"

 protected override void Update(GameTime gameTime)
        {
            
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
                this.Exit();

            Bounce();                                                                        
                      
            base.Update(gameTime);
        }

and my "Bounce" method under "GameMechanics.cs"

namespace Cannon4
{
    class GameMechanics
    {
        const int SCREEN_WIDTH = 640;
        const int SCREEN_HEIGHT = 480;

        GameItem target;
   
        public void Bounce()
        {
            //Bouncer target vekk fra nordre og søndre vegg
            if (target.Position.Y <= 0)
            {
                Vector2 vecVar;
                Vector2 posVar;
                vecVar = target.Velocity;
                vecVar.Y *= -1;
                target.Velocity = vecVar;

                posVar = target.Position;
                posVar.Y = 0;
                target.Position = posVar;
            }
            else if (target.Position.Y >= SCREEN_HEIGHT)
            {
                Vector2 vecVar2;
                Vector2 posVar2;
                vecVar2 = target.Velocity;
                vecVar2.Y *= -1;
                target.Velocity = vecVar2;

                posVar2 = target.Position;
                posVar2.Y = SCREEN_HEIGHT - target.Origin.Y;
                target.Position = posVar2;
            }
        }
    }
}

Why do I get the error "The "Bounce" does not exist in current context"?

At first glance I would say you are incorrectly calling Bounce(). It looks like you're trying to call Bounce() as if it were part of the main program, but Bounce() is inside the class GameMechanics and it is not part of the main class.

GameMechanics.Bounce()

maybe?

At first glance I would say you are incorrectly calling Bounce(). It looks like you're trying to call Bounce() as if it were part of the main program, but Bounce() is inside the class GameMechanics and it is not part of the main class.

GameMechanics.Bounce()

maybe?

no thats not the problem. Cause ive fixed it with removing my folder "Classes" which it lies under... So now its a part on main class.

namespace Cannon4

it WAS

namespace Cannon4.Classes

If you tried it and it didn't work then I don't know. If you didn't try it then you should.

Yeah, I tried it :S

Unless your main class (Game1?) is not derived from GameMechanics you can't call Bounce like that.

You need a GameMechanics instance somewhere. C# doesn't have class-less functions.