The trick to editing more than one record is to use the Edit option under the More Actions button. If you have specific records that you want to change, use the Advanced Find feature of CRM to find the ones you want, then select them on the screen (using the checkbox, or shift-click, or Ctrl-click like you would with other Windows apps) and then click on More Actions and Edit. This will give you a blank form – any field you change there will update all the _selected_ records when you hit the save button. Warning: there is no Undo feature.
The reason I underlined the word “selected” is that all your views are limited to a certain number of records, between 25 and 250, depending on your setting under CRM’s Tools/Options/General/Records per page setting. So, for example If you have 1000 records to change, you will have to hit the Next page icon in your view and select the next group of records and do the More Actions/Edit again.