when i was writting a swift program in xcode. it automatically overwrite the entire old same programs already saved.why it overwrite the entire copy of old programs. what is the reason behind it.how it can be solved?

Are you talking about source code or the binary output?

How old?

Perhaps the old Swift was migrated to new.

Do you have a tested and reliable backup you can pull from?

I use a very basic solution: each day, I copy the whole project directory. And then I can work on the current one.

This helped me several times to find when (and thus where) I had introduced some regression in code that was discovered late after.

Its the Souce Code that is getting overwritten

Consider turning on source control for your project, "git" for example.