Here is what an ServiceStack OrmLite multiple field update with where clause looks like:
Db.UpdateOnly(
new Table_DTO_Object { Field_1 = Val_1, Field_2 = Val_2, Field_3 = Val_3 },
obj => new { obj.Field_1, obj.Field_2, obj.Field_3 },
obj => obj.Id == objId);
How to update multiple fields on a single table row / with a where clause is not immediately apparent from the ServiceStack documentation because they don't have an example with both 1) multiple fields and 2) where clause.
They have an example that updates multiple fields and they have an example of an update with a where clause - really all you need to do / I did is take the needed functionality from each example.