You need to write or use your own code for that. Also, most GPS receivers come with associated software usually includes data logging and conversion packages. Theses converters, however, often convert data into ASCII or text format not Matlab files.
i think you first you need to know that the Rinex is composed of four different data files groups and they are all mostly ASCII format logged:
1. Observation Data File 2. Navigation Message File 3. Meteorological Data File 4. GLONASS Navigation Message File
if you were interested in navigation message files you should first as Malek karaim told you build your own code to separate the header part from the data part and then check the data format.
i have attached a link in which i think is very helpful to revise the Rinex files and more you can use the format you interested in and build a code to extract the useful information for you.