The headword is the present active imperative, third person singular, of the verb u(mne/w (cf. upsilon 112, upsilon 114). It must be quoted from somewhere, probably from the Septuagint version of the Old Testament, where it is found in the typical phrase u(mnei/tw kai\ u(peruyou/tw au)to\n ei)s tou\s ai)w=nas "Let [someone] praise and exalt him unto the ages." (Odae 8.74.2, 8.83.2, Daniel 3.74.2).
[1] LSJ s.v. doca/zw II.