Q: Can i replase memory myself if I still have guarantee?

MBP Retina ... cannot be replaced.

MBPnon-Retina, your warranty will be intact.  Go to crucial.com or macsales.com to see RAM that is available, and you should like the prices as they re lower than Apple's.

No.  Apple posts article on how to do it yourself, if your computer is in fact one that you "can do it yourself" for.  An example of one of these articles is - http://support.apple.com/kb/HT1423.  You would need to know they exact make and model of your computer to ensure you get the correct instructions. 

This link will show the make and model of your computer - https://selfsolve.apple.com/agreementWarrantyDynamic.do.