I'm assuming you googled for https://forum.unity.com/threads/how-to-change-field-of-view-in-vr.451937/

The hacky way to do it is add a couple cameras to your scene, each rendering to a render texture, attached to your headset rig with some eye separation you think is right and whatever FOV you want. Then add a quad to your scene, using a custom material and shader that 1. billboards the quad to the entire view in the vertex shader i.e.screenspace, and 2. in the fragment shader, read both render textures (left/right eye) and lerp between the two using unity stereo left/right variable. Change the render queue of the shader to as high as it goes so it renders on top of everything

It'll triple your draw calls since you're rerendering the scene once for each eye again, and overwriting the"real" view, but should be fine for a personal project.