t1 = Table[{x, y, 0.}, {x, 0, 5}, {y, 0, 5}]
     (* {{{0, 0, 0.}, {0, 1, 0.}, {0, 2, 0.}, {0, 3, 0.}, {0, 4, 0.}, {0, 5, 0.}},
         {{1, 0, 0.}, {1, 1, 0.}, {1, 2, 0.}, {1, 3, 0.}, {1, 4,  0.}, {1, 5, 0.}}, 
         {{2, 0, 0.}, {2, 1, 0.}, {2, 2, 0.}, {2, 3,  0.}, {2, 4, 0.}, {2, 5, 0.}}, 
         {{3, 0, 0.}, {3, 1, 0.}, {3, 2, 0.}, {3, 3, 0.}, {3, 4, 0.}, {3, 5, 0.}}, 
         {{4, 0, 0.}, {4, 1, 0.}, {4, 2, 0.}, {4, 3, 0.}, {4, 4, 0.}, {4, 5, 0.}}, 
         {{5, 0, 0.}, {5, 1, 0.}, {5, 2, 0.}, {5, 3, 0.}, {5, 4, 0.}, {5, 5, 0.}}} *)
     t2 = RandomInteger[5, {5, 3}]
     (* {{0, 5, 3}, {4, 3, 3}, {1, 4, 4}, {5, 3, 0}, {2, 5, 3}} *)
     rplcmntRule = {#[[1]], #[[2]], _} -> # & /@ t2
     (*  {{0, 5, _} -> {0, 5, 3}, {4, 3, _} -> {4, 3, 3}, 
          {1, 4, _} -> {1, 4, 4}, {5, 3, _} -> {5, 3, 0}, {2, 5, _} -> {2, 5, 3}}*)
     t1 /. rplcmntRule
     (* {{{0, 0, 0.}, {0, 1, 0.}, {0, 2, 0.}, {0, 3, 0.}, {0, 4, 0.}, {0, 5, 3}}, 
         {{1, 0, 0.}, {1, 1, 0.}, {1, 2, 0.}, {1, 3, 0.}, {1, 4, 4}, {1, 5, 0.}},
         {{2, 0, 0.}, {2, 1, 0.}, {2, 2, 0.}, {2, 3, 0.}, {2, 4, 0.}, {2, 5, 3}},
         {{3, 0, 0.}, {3, 1, 0.}, {3, 2, 0.}, {3, 3, 0.}, {3, 4, 0.}, {3, 5, 0.}}, 
         {{4, 0, 0.}, {4, 1, 0.}, {4, 2, 0.}, {4, 3, 3}, {4, 4, 0.}, {4, 5, 0.}},
         {{5, 0, 0.}, {5, 1, 0.}, {5, 2, 0.}, {5, 3, 0}, {5, 4, 0.}, {5, 5, 0.}}} *)