The ChangeOrderWeight order rule action will change the weight of the order to the value entered on the Param within the Order Rule. For the Param field enter the Weight (Just the value not the unit as well) you want to set on the order.
Example - Change weight to 1kg if below 1
You want to round the weight up to 1kg if its below 1:
Action: ChangeOrderWeight.
Param: 1.
OrderRuleConditions -
Field: TotalWeight.
Operator: LessThan.
Value: 1.
Then, if an order imports with a weight below 1 it will change to 1kg.