One way is to use VV . It is an open-source and cross platform image viewer. I would suggest using vv to open the image and save it to the requisite format.
Other way is to (In case MATLAB is required)
(i) Use the MITK visualization software to convert from .mha to .raw
(ii) Use the following Matlab toolbox to read the .raw
PS. I have tested neither of them. I am just suggesting untested workarounds.