In the Tanya, Iggeret HaTeshuva Perek 4, the Ba'al HaTanya writes that the statement "there is no teshuva for spilling seed" applies only to "lower teshuva" ("re-attaching the second hei to Hashem's name") but "upper teshuva" (attaching the first hei back to Hashem's name) still works...
What exactly does this mystical idea of "upper level teshuva" imply, and thus what is necessary to perform "upper level teshuva" for spilling seed?