How do I serve notice to leave my rented property? I need to move out early.

You will need to serve notice in line with the terms laid out in your tenancy agreement. Depending on the type of Tenancy Agreement you have you are likely to have a couple of options.

If you have an Assured Shorthold Tenancy Agreement for a fixed period of time, your tenancy will have an end date which permits you to leave at the end of that term. You may also find that you have a break clause in your contract which will allow you to end the contract early, and if you wish to use this you'll need to adhere to the specific terms within the clause (e.g. commonly this allows you to serve a period of notice (1 month / 2 months) at a certain point in the contract to bring the tenancy to an early end)

Alternatively you may be in a 'rolling contract', it is not uncommon for this to be the case where the initial fixed term agreement has ended and you and the landlord have agreed to continue the tenancy allowing you to stay on in the property without renewing the contract with an additional fixed term, the tenancy is known to have continued as a 'statutory periodic' tenancy. In such an agreement there is no end date, and in order to terminate the tenancy you would usually need to give one month's notice to your agent or landlord to leave the property.