To apply for the historic tax class, your vehicle must be more than 40 years & 12 months old (41 years old).

The vehicle will only be used for private/social/domestic.

Submit a V70 or take your V5C to your local Post Office with V5C changes made to the tax class on page 2.

https://assets.publishing.service.gov.uk/media/657065757391350013b03bd6/application-to-change-a-vehicle-tax-v70.pdf 

https://www.gov.uk/historic-vehicles 

To apply for the historic tax class, your vehicle must be more than 40 years & 12 months old (41 years old).

The vehicle will only be used for private/social/domestic.

Submit a V112

https://assets.publishing.service.gov.uk/media/6761486f1ca3ec0a49e18fa0/v112-declaration-of-exemption-from-mot.pdf 

https://www.gov.uk/historic-vehicles