Hi,
It's possible to change the network type for all manholes or selected manholes at once. You can use field calculator for this. This works the same way as in ArcMap. follow the below steps.
1. first select the manholes. then open nodes table from MOUSE drop down.
2. select field calculator by clicking on the attribute heading and type 0 to set Null value, 1 for "Waste water", 2 for "Storm water" and 3 for "Combined" in field calculator.
I have added two images for your reference for step 1 and 2.
For future reference, drop down tables in Mike Urban which have only specific values can be correlated with values 0 to n. for example for Nodes status , you can change all manholes status at once by giving 0 to 10 value in field calculator.
Regards
/Mohit