Navigate to Administrator -> Aktivities
Click on the activity that you want to move participants from, in this example we choose "Informationssäkerhet".
Click on the Participants tab, Mark the field you want to move participants from.
In the Edit dropdown you have the two options, either Move to other activity or Copy to other activity.
Clicking on one of the alternatives will trigger a new popup window. Here you can search for existing activities, you can click on search without filling out the fields which will poplulate every activity available to you.
Mark the activity you want to move to and click on Assign participant to move the participants to the new activity.