My toaster only toasts on one side of the bread...can I replace the toasting elements? It's a Kitchen Aid brand.

There are tow issues here: Bilateral elements (heating strips) of course are necessary for two sided toasting, and you mentioned the Bagel Setting. Are you saying the Bagel setting works with both sides in operation and inoperative when in normal mode? Obviously is in both modes, should only one side function, the element is burnt out.
We need more information.

How to Fix a Broken Toaster - For Dummies

www.dummies.com/how-to/content/how-to-fix-a-broken-toaster.html

Toasters are uncomplicated appliances with heating elements, a thermostat, switches, and plugs and cords. ... If you get a shock when you touch the toaster, it's not grounded. ... What to do when your toast won't stay down ... Take off the carriagelever and front cover and look for a knob, screw, or nut located near the toast ...

How I fixed my toaster - YouTube

? 6:15
https://www.youtube.com/watch?v=QKiJkpuHy-Q
Feb 5, 2010 - Uploaded by paisteboy
Hey I know its been like 5 years since you posted this but how didyou get off the black knobs? I have a similar ... Thanks much - I shall try this on my mother-in-law's Oster toaster! ... But now that side won't stay down, so what to do? .... About · Press · Copyright · Creators · Advertise · Developers · +YouTube.

Because the nichrome wire on one side of the toast has broken.
Its time to get a new toaster.

You definitely don't have the "bagel" button pressed, right? If it is in bagel mode, it only uses the heating elements on one side.

The heating element wire is really thin and there is a great posssibilty of breakage. There is also a spring switch in which the lever locks when toasting. needs replacement.

Go for a new toaster

if u need a manual for a toaster, then u r really in trouble...espcially for the amt of money u paid for it.
ok...
if u need instructions on how to use a toaster...here goes:
When:
A toaster is used to heat slices of bread, bagels, waffles, etc. to a hot, crispy consistency. Most toasters allow the user to set the toasting length to ensure the toasted bread comes out to temperature and crispiness to the user’s likes.
Where:
Toasters should be used on a flat, uncluttered surface near an electrical outlet but far from sinks, or running/standing water.
How:
To toast bread in a toaster, follow these instructions:

1. Place slices of toast, one each, in the slots on the top of the toaster.

2. Find the toast setting (a knob or lever) and adjust to the desired wellness of the toast. (Generally, there is a scale printed near the knob or lever ranging from light to dark toast).

3. Push down the toasting lever (moving the slices of bread down into the toaster between the heating elements.

4. When the toast is finished, the slices will “pop” up automatically.

5. Remove the toast from the toaster.
WARNING:
If the bread should get stuck inside the toaster, DO NOT PUT ANYTHING INSIDE THE TOASTER in attempt to dig it out. UNPLUG the toaster, turn it upside down and shake. If the toast does not fall out, use a plastic fork or knife to gently remove it from the toaster. Using metal utensils could damage the heating elements. NEVER stick anything in the toaster while it is plugged in.
*For further safety, it is recommended that the toaster be unplugged after each use.
I wish u luck and happy toasting.

The other side that isn't toasting has an open element. Usually, the good side will burn twice as hot, and may have stressed that element. You might want to replace both in that slot. make sure you check the voltage rating plate. 208 and 240 volt toasters use different elements.
I just posted the procedure to replace elements, in a similar post here.