Use "Tools" -> "Select Elements". On tab "Nodes", choose "Use These Criteria", value "Label for option "Select", value "Infix Match" for option "Match Type", and enter the word you are looking for as value in the "Label" field. Optionally, choose "Case Sensitive". (This last option is available as of yEd 3.12.)
The same options are available for "Edit" -> "Find".