The quick start guide should give you a good rundown, but basically, you use a rule to set a command to the Command field in the QdabraRules data source, and it runs that command. For SetValue, the command format would be:
SetValue /xpath=xpath of the field to modify /value=value to set to the field
You can use a concat formula to dynamically insert values into the command, so to set a field on the third row of a repeating group to the value of a field called address, you could use a formula like this:
concat('SetValue /xpath=/my:myFields/my:group1/my:group2[3]/my:addressField /value=', address)
Likewise, you could concat in another field value to dynamically change which row is modified:
concat('SetValue /xpath=/my:myFields/my:group1/my:group2[', rowToModify, ']/my:addressField /value=' address)