If you want to permanently change the default program a specific file type such as *.htm is opened with, then you will need to change the file association for that file type in Windows.

To do that, open a Windows Explorer session on the PC.  There are several ways you can open a Windows Explorer session. The two easiest methods are to:

Right click on the Windows Start button and then select Expore.

or,

Cick on Windows Start, then Programs, then Accessories and finally, Windows Explorer.

When the Windows Explorer session is open, click on the Tools menu item at the top of the page.

Next, click on the menu option named Folder Options.  You will be presented with a panel like the one shown here:

Next, click on the File Types tab and you will be presented with a panel like the one shown here:

Scroll through the list to find the Windows file type you are interested in and then click on it to highlight it, and finally click on the Change button in the middle of the page.

You can browse through the list of known executable programs or you can browse your hard drive to find the specific executable program you wish to use for this file type.

Once you make a change here, this will become the executable program that is assigned to the chosen file type. This will be the program used to open all files with that file type when you select the *FILEASSOC option on the PC-Engine commands.