There are times that you may need to change the assigned user of a contact, company, deal, or case, for instance, the assigned user left the company or the item needs to be escalated to a higher-up user.
If you need to change the user on a single item, the easiest way to do this is with inline edit. Above the name of the item (contact name, etc.) is the assigned user. Hover over the field, click the Edit button, change the user and then click outside the field. The new user will get an email and a notification that they have something new assigned to them.
If you need to change all of the items to a new user when the user leaves the company, you can bulk update the assigned. Filtered the data based on the assigned user, select all of the items, and then use the bulk update menu for Assigned User, select the username, and click Update.