Here the 1874 RLDS edition switched the order of the perfective have and the subject I in the environment of the fronted direct object this (in accord with the biblical style). In the 1908 RLDS edition, the original word order (“this I have done”) was restored to the RLDS text, probably by reference to 𝓟.
Elsewhere in the text, we get three examples of the inverted order with fronted this and the perfective auxiliary have:
However, there are also three other examples with the noninverted order:
Either order is possible, so we follow the earliest textual sources in Helaman 5:6 and maintain the noninverted order.
Summary: Retain in Helaman 5:6 the noninverted order “this I have done”, the reading of the earliest textual sources.