The How to Rent guide
For all new and replacement tenancies, landlords must have served tenants with the current How to Rent guide before a Section 21 notice can be served.
At the start of each 'replacement' tenancy, if the booklet was previously served then there is no requirement to re-serve the booklet unless it has been updated
Get the guide
Here is the how to rent guide from GOV.UK. On that webpage it will tell you when it was last updated.