Hi Janet, Here's a video to remove window and put in a door. It will be the same principle for the door you want to install. https://www.youtube.com/watch?v=vdd9NOttPVU
Here's a how to video for bi fold doors: https://www.youtube.com/watch?v=_kv_GNj6i_Q
If you’re enlarging the width of the opening, you’ll be replacing the header and that will need to be re-sized. Not just longer but thicker/higher.
If the width is increasing then your vertical framing, ie the king studs and crippled will have to be replaced and new hold downs will be required to go into the foundation.
Both of those put you in a “threshold” condition, at which your local Building Department may require a Building Permit (to ensure it’s done in a safe and Code-compliant manner).
Flashing and counter flashing need to be addressed.
There’s a high probability that you’ll find unforeseen conditions.
If none of this makes sense to you, you’ll need hire an experienced contractor and tell him you want to help him, to save money.
Break the caulking off around windows and unscrew all screws
I found a video on YouTube I think you might like...