Hi there @Shahnam1 - thanks for using Dropbox and welcome to the Community!
Your files and folders won't get affected when you cancel your subscription. Only syncing will stop and you won't be able to add more files to your account, if you're over the quota that comes with a Basic plan that's 2 GB.
Once your account gets downgraded to the Basic plan, you'll be able to upgrade to the plan you want normally.
I hope this helps!