Can no retrieve because is md5 crypted, but you can change by someone know.
Option one
1- backup
2 - Install geany
3 - With openkm stopped try to open the database file, into jboss/server/default I do not remember the exact location because OpenKM 5.x is an older version what we consider deprecated, I remember some folder called data/hypersonic I'm not totally sure. There you will got a file called okm_app.script or similar ( open with geany ).
4- Looking for OKM_USER you'll see sql inserts, one of these should be okmAdmin change password to '21232f297a57a5a743894a0e4a801fc3' this password value is "admin".
Option two
Stop openkm
With some utility like datastudio ( aquafold ) connect to hsql database and then change the password to '21232f297a57a5a743894a0e4a801fc3' this password value is "admin".