nii files follow the NIfTI image format. You can use the niftiread function to read these files. MATLAB also offers other functions like niftiwrite and niftiinfo. All the linked doc pages have examples that should help you get started. You can use try using volshow or the volumeViewer app to view your data.
Here are two articles that talk about viewing/displaying MRI data (3D volume):
You can try adapting techniques from these articles.
Here's a popular File Exchange submission for working with NIIfTI and ANALYZE images: https://www.mathworks.com/matlabcentral/fileexchange/8797-tools-for-nifti-and-analyze-image
