I am come down (καταβεβηκα). Perfect active indicative of καταβαινω. See on John 6:33 for frequent use of this phrase by Jesus. Here απο is correct rather than εκ with του ουρανου.

Not to do (ουχ ινα ποιω). "Not that I keep on doing" (final clause with ινα and present active subjunctive of ποιεω).

But the will (αλλα το θελημα). Supply ινα ποιω after αλλα, "but that I keep on doing." This is the fulness of joy for Jesus, to do his Father's will (John 4:34; John 5:30).