What exact model and/or what exact model of brake/shifters does it have - makes a big difference what you can do.
However, I've developed a bit of a back problem and want to change my handlbars for straight ones as the reach is too much with the drop bars (braking). Is this easy to do?
Usually its fairly problematic (or at least it needs a LOT of modification) to do what you want - convert drop handlebars to flat.
Often, the easiest compromise is to adjust/modify what you already have, e.g. :
Raise your existing handlebars up if your current headset spacer configuration allows it.
Or perhaps buy a shorter stem. Or buy one with a shorter reach AND something like a 45deg rise. (often, existing stems have a slight angle which is often fitted downwards, so you can simply remove and refit the existing stem 'upsidedown' which can make a noticeable difference).
For back issues, often only adjusting the existing bars an inch or less can make a massive difference.
If you decide to rotate your existing bars around as others have suggested, note that you yourself need to have had a prior frontal lobotomy to make this an acceptable solution....
;-)