oh. well. um. yes, you go to vertexes, you turn only 1 geoset on at a time, and left click a surface area, and click the button on the side called ''delete triangle'' do not delete any thing else, then they become points un attached to each other with no triangles. just do that over and over til you have all the triangles deleted, and make your own new triangles, in this case, the shape of trapazoids, or is it tetchrehdron? the /`_`\ shape inbetween them, and make the new triangles the way you want them, for example, with a big square hole in the center.
next, you'd need to fix the UV wrap, by going to the UV wrap section, and if it has any image on it, which you could yes then fix slowly once you figure out what points are where in the uv wrap.
if it doesn't have an image, then it doesn't matter too much.
it would help if I knew what model were even talking about here, this ones pretty minimal. is it mostly particle effects?