For .uk domain names
Please get the IPS-TAG from the company you want to change to and then see the article on how to change the IPS-TAG please.
For international domains, ie .com .net .org .biz .info
Please retrieve the auth code (there is an article on how to do this in the domains section of this knowledgebase) Then contact your new provider and they will give you instructions on how to process the transfer including what to do with the auth code. The process will vary slightly from company to company but once you have the auth code you are in control of the transfer process.