Hi,
After a few weeks I decide to buy the original template.
From what source you had files before buying ?
... How I will install the original files from the template and not change my styles...
If you made customization in template you should know exactly in which files you made it.
So copy those files in local disc (backup) , install (current version of) template and copy again only those files which you changed or add changes again. Probably you thought (edit) only one or two css files, so there won't be any problems to upgrade.