Hi
Thanks for the question but unfortunately, extending a trial subscription is generally not possible. as you said that you are using it for research I suggest you to upgrade your plan from a free trial to a pay-as-you-go subscription.
For more details and upgrading the plan and about the pay as you go subscription kindly refer to the documentations below.
https://azure.microsoft.com/en-us/pricing/
https://azure.microsoft.com/en-in/pricing/purchase-options/pay-as-you-go