Umm... Paint?
I'm assuming that you genuinely don't know that Windows can natively convert lots of files between different formats.
Right-click the JPEG, choose Open With > Paint
Once opened in Paint, do File > Save As...
Choose 24-bit Bitmap in the 'Save as type' drop-down box
If you wanted to batch convert, then probably
IrfanView is your best bet. However I'm sure that if you Googled for 'JPG BMP converter', you'd find a million others...