Access uses the Mod keyword as the modulus operator.
DoCmd.RunSQL "UPDATE OTCValidation LEFT JOIN Option16 ON Option16.[Plan Code]=OTCValidation.[Plan Code] SET OTCValidation.[Step 1 Actual]= Option16.[RXOTCIND] WHERE Option16.ID Mod 2 = 0 AND OTCValidation.[Plan Code] is not Null"
DoCmd.RunSQL "UPDATE OTCValidation LEFT JOIN Option16 ON Option16.[Plan Code]=OTCValidation.[Plan Code] SET OTCValidation.[Step 2 Actual]= Option16.[RXOTCIND] WHERE Option16.ID Mod 2 <> 0 AND OTCValidation.[Plan Code] is not Null"