You can't downgrade to the trial plan during the trial, you would have to wait until your trial is finished and then change to the free plan. This will however, mean that your services will be down between the time of the trial ending and you downgrading.

If you want to avoid the downtime mentioned, the Hobby plan ($5/mo) is a good option, but otherwise, you'll just need to wait for the trial to lapse to hit the Free tier.

i need to know when the trial ends how can i change my plan to free one

my trial ended 2 hrs back and not able to access it. How to downgrade it?

You are going to need to wait until the trail ends, then you can switch to the free plan in the billing menu of the settings. If you are still experiencing any confusion, please send me some screenshots with any important info blurred out, and I can help navigate.