I have recently bought a Canon EOS 550D. In the Manual Mode, I am unable to change the exposure level. What am I missing here? I am sure there must be a way to change it.
Based on experience, I am certain you are confusing exposure and exposure-compensation.
Exposure is a product of 3 parameters: ISO, shutter-speed, aperture. In Manual mode, you control all of these and doing so sets the exposure. @jrista shows very clearly how to control each of them. For how they relate, see What is the exposure triangle?.
Exposure-compensation (EC), marked on most cameras including yours by a ± sign, is an offset from the exposure the camera is computing in automatic (P, Auto) and semi automatic mode (S, A). In all these modes, the camera computes an exposure (based on the metering mode) and EC is how you tell the camera to adjust the computed value by a certain amount.
In other words:
- In all modes except manual: The camera computes an exposure and you use EC to tell it to make things brighter or darker relative to its guess.
- In manual mode: You set the exposure exactly using Aperture, Shutter-Speed and ISO. There is no offset the camera can apply because all the parameters have been fixed by you.
To change the exposure settings in Manual (M) on any of the Canon Rebel series, you use the main dial. Its near your shutter button, but a little more towards the back of the camera. You can't miss it, its a notched roll-type dial. By default, when in manual mode, rolling that dial will change your shutter speed. Roll one way to increase, roll the other way to decrease. To change your aperture, you use the same dial, however you must also press and hold the Av (+/-) button on the back of the camera while you roll the main dial. Pressing and holding the Av button will switch to aperture adjustment mode, instead of shutter adjustment mode. If you hold the camera right, that button should be easily accessible with your thumb. It should be noted that usually, the camera adjusts shutter and aperture in "1/3rd stops". This can be changed via the camera menus to "1/2 stops" if you so prefer. However you have it configured, one "tick" of the main dial represents one of those values. So by default, three "ticks" of the main dial would adjust shutter or aperture by a full stop.
To adjust your ISO setting, you press the ISO button, which is again a little farther towards the back of the camera from the main dial. You press this button once to enter ISO adjustment mode. You can then either use the arrow controls on the back of the camera to select an ISO, or use the main dial again to "roll" through the ISO options. When you have selected the ISO you desire, press the ISO button again to actually set the selected ISO.
Finally, while not specifically a part of exposure, another common control is AF point selection. This can be achieved by pressing the the increase magnification button on the upper right back corner of the camera (looks like a blue magnifying class with a + in it). Once in AF point selection mode, you can again either use the arrow keys on the back of the camera to select an AF point, or use the main dial to roll through them. Pressing the same button again sets the selected AF point (or points, if you select them all.)
In manual mode, you are in control of the exposure factors - shutter speed, aperture, and ISO. To change the exposure, you'll vary these three factors in your camera - there are controls on your camera to change each of these factors independently. There shouldn't be one particular button or feature that changes them all while in Manual mode.
It's hard to tell what you are missing if you don't tell us what you've tried.
In manual mode, you can adjust shutter speed by turning the control dial (a rotating disc with ribbed edge sticking out of the body) behind shutter button. To adjust aperture, hold Av/exposure control button and turn that same dial. To adjust ISO sensitivity, press the ISO button behind the control dial and use the control dial or arrow buttons to choose another ISO (look at upper left corner on LCD for current setting).
When you change exposure, viewfinder status bar and LCD should show how much the new exposure is different from metered exposure, or blinking +/- if the difference is larger than 2 stops. Also, exposure parameters change on LCD and in viewfinder status bar.
If turning the control dial does not change the parameters, your camera probably needs repairing.