How can I convert from an overhead door to a man door?
Assuming that the walls are standard wood construction, with dimensional lumber, pulling out the door and putting in a standard entry door is fairly straightforward.

First take out the rolling door. It's best to do this in the reverse order that it was put in. Carefully remove / release the tension on any springs or lift assists that may be in place. If the door is a series of panels bolted together with hinges, start at the top and remove the bolts holding the top panel to the one down, and remove the rollers at the sides of the door. Set the panel down, and repeat with the next panel down.

Once the door is out of the way, remove the track and any other door bits.

Then you have a choice. You can leave the already framed / trimmed opening and put in a wall to house the new door set back a bit, or you can remove the trim and match the existing siding. 

In either case, you'll have to frame up a smaller wall to fit in the opening with a new door header / opening, place some sill gasket material down, and fasten it to the slab and existing opening.

Once the temporary wall is up, you can sheath it, leaving the new door opening clear, and place siding over that.

Install the new door and hardware, and the space is enclosed.

You didn't mention what the inside of the structure is like. If you're considering turning it into an office, you might consider an air conditioner, drywall, and nice inside accouterments.