In order to change the name of one of your subprojects you have to close the Master Project and open only the Subproject file from where it is saved (clouds or on local device).

After that, go to the File menu – Options – Project Info tab and from “Project Name” field, change the name of the Project and click “OK”. Save the file and then open the Master Project and expand the subproject whose name you just changed and it will appear with the new name.