I Was willing to purchase a full version of Lightroom and by mistake I've bought only the upgrade to Lightroom 5. What can I do to give back this upgrade in order to buy the full version?

Return, cancel, or exchange an Adobe order

Which part are you not clear on?

This is a user to user forum so the responses you usually obtain here are from users like yourself, you should contact Adobe support directly or the source supplier of the upgrade you purchased.