Hello,
I believe what you are looking for is the 'Start Loop Through Grid' action. (Don't forget, it has an corresponding 'End Loop Through Grid' action). With the loop action, you can find the records you have checked. For each record found you have an 'Update Field In Table' action that updates the record with the correct information.
We do have an cuatomization example that is relevant. It's for mass deletion but can be modified to suit your needs here. You can see how to setup a loop through a grid.
You can do a mass update with the import update function. This will not give you the interface for a user to update records but you can update many records at once to get yourself started.
Here is the Start Loop Through Grid function from the FirledService_WorkOrderList screen and the "Invoice selected..." action. Take a look to get a sample.