Hello Karthik,
To change the compiler in the mex command, set the varname variable. varname for the gcc compiler is GCC, in uppercase letters. For example, if the currently supported gcc compiler is version 4.7, and it is installed in the /usr/bin/gcc-4.7 folder on your system, to build timestwo.c, type:
copyfile(fullfile(matlabroot,'extern','examples','refbook','timestwo.c'),'.','f')
mex -v GCC='/usr/bin/gcc-4.7' timestwo.c
Setting the compiler using the mex command does not change the system default compiler.
Also, refer the below link to downgrade GCC in Ubuntu:
