A BMP is not an HTML file.
A BMP is a picture.
An HTML is a document file with pictures, text, etc.
You shouldn't really be doing it in the first place.
In case you still want to change it.
You can do it by simply renaming the extension.
A. Normally Windows Explorer will not show the extension of known file types however you can force Explorer to show them:
1.. Start Explorer
2.. From the View menu select Options
3.. Select the 'View' tab
4.. Unselect the 'Hide file extensions for known file types'
5.. Click OK
HTH,