Every membership that you assign to a client, will have an (initial) end date on the contract. This date is formed by the conditions of the membership, specifically the duration that you've set the membership for. For example, if you set the duration of a membership on 12 months, the end date will be set for 12 months after the start date. This also applies for memberships that will be automatically renewed. The end date of the contract will move up as soon as the initial term of 12 months is up. This means that you do not have to do anything to have the contract renewed, only if you want to stop the contract is when you will need to take action.
To change the end date of a contract, just go to the client's profile, go to the tab products and click on the membership. This will open up the client's membership contract.
If you want the end date of the contract to be earlier than the date that's on it now, just remember that you can't go back further than the last invoice that was created. For example: If an invoice is already created for the month of October, you will not be able to change the end date to September. This also means that if someone paid the full yearly amount in one time, you will not be able to set their end date before the year is up.
If you want the end date to be set after the initial one, you should make sure to cancel the contract after you've changed the end date. Example: If someone comes up to you and tells you they will want to end their contract in 8 months, you can set the end date on that date and cancel the contract. This means the membership will still be active until the set end date, but it will not be automatically renewed after.