Can i assign a category to many at the same time

You can search for all of a certain payee or category by typing in the search box top right of the register. Click the magnifying glass icon to restrict the search to one column. You can also use a filter to limit the dates or choose only last downloaded transactions. Then...

Select the transactions you want to change either using Edit > Select All, shift-click sequential selection) or command-click (non sequential). 

Then click menu File > Get Info, enter the new category and click OK.

Note that this won't work for split transactions, but if these are newly downloaded, there won't be any splits.