To change the owner of a deal from the Deal Grid first thing you should do is select the Owner column into the Grid:
and then filter out that column by the Owner you wish to change:
If you find the deal you wish to change just hover over the colored circle (with the first letter of the deal) you will see a checkbox:
You will then see the Bulk Action dropdown (to the left of the Search bar):
Tip: You can use the Bulk Actions to do multiple deals at a time and you will just need to check the check boxes.
Clicking Change Owner will show you the Change Owner popup which you can select the owner for the deal:
Once you have selected the new Owner just press Save.
Now when you go into a Deal Details page that new owner will be shown in the bottom right corner and when you hover over the far right icon (to the left of the + ACTIVITY button in the Action Bar at the bottom) it will show who the owner is.