Sometimes you may have duplicate companies and want to consolidate deals into one company.  

To change the company the deal belongs to, navigate to the deal details page and click the pencil icon next to the 'Company' text.

An autocomplete box will appear where you can begin typing the new company's name.  Note this company must exist in the system already.  Enter and select the new company's name, then click Save in the upper right.

Tips: When consolidating/merging deals to one company with the same name, rename one of the companies to something else.  For example, if there were two Tom's Bar, rename one to Tom's Bar DUP to make it easier to select and identify which company is which.