Synology DS1821+ Hardware Installation Manual page 20

b Attach the fan case (with the fan cable within the fan pointing at the bottom-right corner) to the new fan, then
tighten the screws to secure the fan case.
c Plug the fan cables into your Synology NAS as shown below.
1 2
6 Replace and tighten the four screws removed in step 3.
20 Chapter 3: System Maintenance