See "How do I edit my research item's details?" in https://explore.researchgate.net/display/support/Reviewing%2C+featuring%2C+and+editing+your+research for instructions. However, ResearchGate just recommends to add abstracts, you are not obliged to to do. But it can be indeed useful to add abstracts so that potential readers will know better about the content of your publications. It may help to get more readers.

You need to summarise each section of your publications in one or two sentences and combine them into a cohesive paragraph that describes your research :)