See the full guidance at: www.gov.uk/government-authorised-exchange
You must have a Tier 5 sponsor under the scheme of relevance to you. Your Tier 5 sponsor must issue a certificate of sponsorship (CoS) to you before you can make your immigration application.
Tier 5 sponsors are listed in Appendix N of the Immigration Rules of Tier 5 (Government authorised exchange) sponsors.
You must be able to show that you have held funds of £1,270 in an account that is acceptable to the Home Office for a period of 28 consecutive days. Alternatively, you do not have to provide any evidence of funds if your Tier 5 sponsor is 'A-rated' and is willing to confirm on your certificate of sponsorship that it will maintain and accommodate you up to the end of the first month of your employment. The sponsor may limit the amount but it must be at least £1,270.
Applying inside the UK
You must apply online via the gov.uk website at www.gov.uk/government-authorised-exchange/switch-to-this-visa
We suggest you seek the help of an immigration solicitor when making your application. Any immigration solicitor you choose should be registered either with ILPA (Immigration Law Practitioners’ Association) http://www.ilpa.org.uk/ or OISC (Office of the Immigration Services Commissioner https://www.gov.uk/find-an-immigration-adviser.
If you have questions before you make your application, you can speak to an International Experience Adviser.