I am sorry I meant by downconvert downsampling which is the opposite of upsampling and you can think of it as a way to represent the pulse shape in MATLAB and thats how you will use the Nyquist pulse shape , and use it again in the RX as Matched Filter,,, or if not neccessary ( in case that everything is perfect , eg no ISI ) you can neglect ( 3&5 ).
How to do upsampling :
you can use Matlab command : "upsample" which is simply add number of zeros after each value. you can read more about it from MATLAB help.
I hope that helped