Prerequisites:
Be a distributor and have an "owner" role on the STS.
To modify the rental duration of a container contract, first go to the STS, then click on "Contract" on the left side of your screen and select the "Containers" tab, as shown in the example below.
Then click on the pencil icon next to the container contract you want to modify. You will then be taken to the page below.
You can then modify the end date of the container contract. Don't forget to validate at the bottom right once the modification is made.
For a step-by-step procedure, refer to the attached file below.