As facebook provides the facility to save posts, videos and images in Saved Items. It also gives option to create collections or folders so that you can differentiate different categories of post according to your understanding.
For example if i have already created a collection named as "Funny Collection" and want to rename it then following steps would be followed.
Open the collection in your facebook app on smartphone.
There is an option"More" in the collection page. Click on it.
Clicking on it will give you different options. I want to rename collection so i will click on it.
Now you need to enter the new name. So i rename it as "Funny Clips" and hit OK.
Finally your collection name is successfully renamed.