This Salesforce Help article walks you through it. It tells you to use Data Loader but you can use Demand tools as well. Also there is a ink at the bottom of the artilce to a You Tube Video on it. https://help.salesforce.com/s/articleView?id=00038523...
Hoping the community will be good for me to learn Demand Tools. I am used to my old stadbys of Data Loader and Workbench so I have not yet embraced this tool but looking forward to trying to use it more now that there is some community support.
For your first item, we have recently done what you are asking about in our org. Unfortunately there is not an "easy" way as the Task/Activity object can be tricky. You will need to prepare your data frst with the User ID of the asignee/lead owner as...
