Simply click on 'Choose a new mem', and then on the left arrow in the memstream. You will be presented with all the other mems associated with this specific item that other users have created. Click on the one you like the most and that will become your new mem for this item.
Create a mem
Edit a mem I have created
Points given for mems
What do I do if I find an offensive mem?
Delete a mem
How to create a mem in the Android App?