Family Historian User Group

How to reverse 'from' and 'To'?

Once you have selected the records to be merged, you can select one of them in the right pane and the (smallish) up / down arrows in the top right of the window become enabled - just use these to move the records in to the order you want.

Not sure exactly what you are asking.

Brent's advice is good for Edit > Merge/Compare Records.

Maybe you are asking about File > Merge/Compare File.

If talking about whole GEDCOM, then instead of starting with Project A and merge from Gedcom B, you must start with Project B and merge from Gedcom A.

Perhaps you are talking about specific fields to Keep or Delete.
Generally, most fields are kept by default.
So to delete any item select it in either Current Records or New Records pane, and click the Discard button.
To reverse that decision, select the struck-out item and click the Retain button.