You have to use loop and achieve.
Lets say the collection like this:
Filed1|Filed2
1|2
2|3
3|4
Flow:
- Start
- Loop Start and choose the Collection
- Calc Stage
- In the expression area put [Collection.Field1] and Store In Collection.Field2. This copies Value from Field1 to Field2 in the same collection
- Loop End
- End
After executing the above flow, the collection will be like:
Filed1|Filed2
1|1
2|2
3|3