Hi. newvibe here
So far i have designed a html website.
I did find membership website softwar ,they will install it for me, thanks god.
Now i need to "to rename my .htm/.html files to .php" i have no idia how to do it.
Could you tell me step by step how to do it
I probably need to read some kind tutorials that help me to understand ?

Thanks in advance.

Renaming the files is as simple as it sounds. It's no different than renaming a file on your computer. The details may depend a little on the software you're using to access the file on your server, but more than likely you would just right click on the file name and the context menu that opens will have an option to rename. Then would would simply type replace the .htm or .html with .php

However

Are you sure you'll need to rename your files. There's a good chance you won't. A website can have some pages that are .html and others that are .php. All that the .php is doing is to alert the server that before it sends that page to the browser it needs to run the file to a php engine so any php code is processed. Unless the pages in question actually have php code on them you don't need to change the extension. If you give a page without php code on it a .php extension it's still fine. The php engine won't see any php code to process and just send the file as is back to the webserver. It's an extra step though that doesn't need to happen.

The membership application will have php code so its files will need to have the .php extension, but they will already be using that extension. Your pages may not need to change unless you've specifically been told they do. My guess is they don't. But if you do need to change them it really is a simple as right clicking and renaming. There's nothing special you have to do.

Go to your hosting control panel file manager (I suppose you can find one), and append to file .htaccess in / (if the file is not there already, create it)

AddType application/x-httpd-php .html .htm

Adding the above directive will execute all .html and .htm files as php, achieving the same result as renaming them into .php on most default configurations.