I have seen many times people asking this question to solve this error:Could not load file or assembly 'xxxx' or one of its dependencies. The process cannot access the file because it is being used by another process. (Exception from HRESULT: 0x80070020)
The top most reason for this error is that you are trying to access your site while the dlls are being updated on that site.When you are overwritting the existing files the copying mechanism takes an exclusive lock on the files (dlls). So it is better to have app_offline.htm page at the root of the site while you are overwriting the dlls.