Exodus 20:5 (King James Bible) thou shalt not bow down thyself to them nor serve them
The initial words and again are Abinadi’s, not a part of the quote itself. Abinadi is simply moving on from the previous commandment that had earlier triggered a lengthy exchange between him and king Noah’s priests (namely, Exodus 20:4, cited in part in Mosiah 12:36 and then given in full in Mosiah 13:12); notice, in fact, the introductory clause at the beginning of verse 12:
One additional difference in Mosiah 13:13 is the use of the preposition unto in the Book of Mormon text, while Exodus 20:5 has to. It is possible that the original Book of Mormon text also had to and that unto is a transmission error. Interestingly, the parallel quote of the Ten Commandments found in Deuteronomy uses unto rather than the to of the Exodus version:
Since this variation between to and unto actually occurs in the biblical text, it seems perfectly reasonable to retain unto in the Book of Mormon text. Furthermore, there are many different expressions in the Book of Mormon text where either to or unto can occur. See, for instance, the discussion regarding the phrase “pertaining (un)to righteousness” under 1 Nephi 15:33 or the phrase “hearken (un)to X” under 1 Nephi 16:3. In other words, the most plausible conclusion for Mosiah 13:13 is that the invariant reading of all the (extant) textual sources—namely, unto—is the original reading.
It should be noted here that Abinadi is citing Exodus 20 rather than Deuteronomy 5. Wherever the two biblical versions of the Ten Commandments differ, the Book of Mormon text almost always agrees with the Exodus 20 version: namely, in 32 out of 33 differences. Only here where the Book of Mormon reads unto does the text agree with the Deuteronomy version. This single agreement seems accidental. There are 14 places where the Book of Mormon text in Mosiah 12–13 varies from Exodus 20, all in minor ways except for one phrasal deletion in Mosiah 13:19 (see under that passage for discussion); and of those 14 differences, only this one involving (un)to agrees with Deuteronomy 5.
Summary: In Mosiah 13:13 the initial words and again are Abinadi’s, not a part of the biblical quote itself; the preposition unto is an acceptable variant for to since in many different expressions the Book of Mormon text allows variation between to and unto.