My phone's internal memory is full so I'm planning to reset my entire phone in order to update my whatsapp, however I want to keep my old conversations.
How do I retrieve my WhatsApp conversations after it has expired?
You can use Helium backup to backup all your chats, groups, etc. Moreover, this app can be used to backup all the applications on your phone.
You can store this backup on your PC, and when you reset your phone, you can upload the whole backup folder on your SD card and initiate a restore from Helium app.
If you are not rooted, you will need its desktop application to backup your phone. It's pretty simple and reliable. You can refer to Helium Wiki in case you get stuck.