i made this site in wordpress 3..1 version also it work fine but in error my wordpress version is up-to-date so site theme is default set what exactly i'm able to do in order to get these version back? thanks

Should you just modified the default theme then I'm sorry to need to tell you just how your changes happen to be lost. When WordPress does an update it overwrites the core WordPress files. If you wish to produce a theme in line with the default theme you have to create a copy from the default theme, provide a distinctive directory title and alter the theme particulars in fashion.cascading stylesheet

eg. If you are using twentyten then copy the entire twentyten directory and relabel it to some appropriate title. Then in fashion.cascading stylesheet discover the following lines and alter them:

Theme Title: Twenty Ten switch to Theme Title: Your Theme Title (just like directory/folder title)

Theme URI: http://wordpress.org/ switch to Theme URI: Your website URL

When the default theme continues to be overwritten there's not a way to undo this.