Here the typesetter for the 1874 RLDS edition made an accidental change, replacing this with his in the phrase “by this band”. The RLDS text has retained this secondary reading. (For another example where an 1874 error has been maintained in the RLDS text, see the nearby discussion regarding the missing again in Helaman 2:1.) For another instance where the 1874 typesetter replaced this with his, see under Alma 18:34 (in that instance, the 1908 RLDS edition restored the original this to the RLDS text).
The use of his does not work well here in Helaman 2:6 since the nearest possible antecedent for the his is “one of the servants of Helaman”, not Gaddianton or Kishcumen. Even the preceding nearest possible antecedent, the pronoun he that refers to Kishcumen (“as he went forth towards the judgment seat”), may not be the referent for this secondary his since the preceding text refers to Gaddianton as having taken over as the leader of this band which was originally Kishcumen’s:
The instance of his band in verse 5 of this passage is potentially ambiguous and could refer to either Gaddianton or Kishcumen, although I would normally read the his as referring to the preceding he, which does refer to Gaddianton (“he would grant unto those which belonged to his band”). Yet later in verse 8 the band is unambiguously referred to as being Kishcumen’s:
In any event, if his were correct here in verse 6, we would be required to skip at least the reference to Helaman’s servant and perhaps even the reference to Kishcumen in order to interpret the his as referring to Gaddianton. The use of this is definitely correct here in Helaman 2:6.
Summary: Maintain the original determiner this in Helaman 2:6 (“those plans which had been laid by this band to destroy Helaman”).