You can use this command to save the Namespace image to the disk.
hdfs dfsadmin -saveNamespace
But before merging the namespace, you need to make sure that the NameNode is in safemode. If your NameNode is not in the safemode you can execute this command:
hdfs dfsadmin -safemode enter
During the safemode, you cannot alter or edit anything in the HDFS. You can only perform read & list operations. This is done because while merging the edit logs & the fsimage, there should be no changes in the HDFS, or the changes made will be lost.