Nikon D90 grid change?

I want the grid to be shown on my viewfinder but I don't want it to be split in 4 but in 3 (I'm amateur and want to practice the rule of thirds a little bit). I own a nikon d90… Any ideas how to change it?

What does the manual say about it?

I don't think you can change that but you can just imagine it this way.

By the time you have learn how to pre-visualize your photos before shooting them, you will also have no need for a "rule of thirds" grid in your camera. For now use the vertical grid lines to approximate the rule of thirds in the vertical plane and then using your eye, approximate the lateral rule of thirds position of your subject.

NOTE: You can for now, use the rule of thirds grid found in Photoshop CC to further refine your images and get used to seeing the proportions the rule of thirds shows you on your computer screen

Instead spend some time on the link below and learn how to use the elements of design and composition.

Those of use who learned the fundamentals of photography using film and no such grids or other helping devices to aid us as well we learned how to compose brilliant images. That meant that we hand to know how to compose amazing images in the camera and be patient as we waited for the film to be developed and the negatives printed to see if we had amazing shots or just so, so shots.

Be patient and take your time. There's NO time limit set on your creativity and it is important to learn shot discipline so you do not have to spend hours sitting at your computer editing (sorting) the best of your images from the rest. The goal here is to produce excellent images, each time you press the shutter release and this requires much thought and consideration before you create an image

Just switch off those nasty lines and instead focus on your compositions. Rule of 3rds is a good place to start but do not get enslaved by it.