iMac Late 2015 RAM Upgrade 21.5" Quad Core 8 GB

HI all, I've just purchased and started using today my new iMac as stated above, but for what I'm doing its not coping with the 8GB of RAM. This wasn't mentioned to me at the time of purchase from PC World that I couldn't upgrade it. Does anyone know if it's possible for Apple to do it, as I can't take it back now to swap it due to having put all my programmes on it ? Or any ideas ? Before anyone says it's my fault for not checking, I know, but we all make mistakes ! Thank you