I am unable to remove the back to convert this car seat to a booster.

Watch these 2 instructional videos on how to convert the Evenflo Symphony car seat here:
http://www.evenflo.com/support.aspx?id=452
http://www.evenflo.com/support.aspx?id=460

This car seat does not convert to a booster. It becomes a "Belt-positioning booster". The back and headrest remain while using as a booster. You remove the 5-point harness and use the car's shoulder strap seat belt.
The belt positioning is done by the plastic parts at the top on either side of the seat.
Following is the link to get the instructions for this seat: http://www.coscojuvenile.com/usa/eng/Instructions

Turn the car seat over and remove the pin that holds the base in place. Then remove the belts and the buckle. voila, a booster seat.

The base is what makes it a booster seat. Some seats the back is removed but if yours is the one in the picture, it looks like you have a belt positioning booster seat (much better than a regular booster). You would remove the harness straps completely and place the automobile's seatbelt through the appropriate slots on the side of the carseat up by the shoulder/head and feed it across your child rather than through the seat. For best use, look in your owner's manual for instructions. If you no longer have it, generally you can download one on-line from the manufacturer at no charge.

Take the base off if it has one, either clips, screws, or a metal rod holding it on. Take the belts out. They should slide off the clip at the back of the seat, then unthread belts from buckles. The buckle/strap between the legs usually has a metal keeper below the seat. To remove you need to bend/twist the keeper until you can push it UP through the seat and remove the belt/strap. It is now a booster seat for use with a lap/shoulder belt in the car. Store all the parts you removed in a ziploc bag for when your child outgrows it so it can be reused.

The Seat NOW is SIMPLY a BOOSTER, And Thus 5 Point Can Be Removed, and The VEHICLE SEAT BELT Can Now Be Used.

u need pliers! there is a long pin in the bottom goes from left to right. pull it out and bottom detaches

Types of car safety seats at a glance
The chart below is a quick guide on where to start your search. However, it’s important to read more about the features and how to use your car safety seat.
Age Type of Seat General Guideline Infants Infant seats and rear-facing convertible seats All infants should always ride rear-facing until they are at least 1 year of age and weigh at least 20 pounds. Toddlers/Preschoolers Convertible seats It is best to ride rear-facing as long as possible. Children 1 year of age and at least 20 pounds can ride forward-facing. School-aged children Booster seats Booster seats are for older children who have outgrown their forward-facing car safety seats. Children should stay in a booster seat until adult belts fit correctly (usually when a child reaches about 4' 9" in height and is between 8 and 12 years of age). Older children Seat belts Children who have outgrown their booster seats should ride in a lap and shoulder belt in the back seat until 13 years of age.

Better yet, please use this manual for clear instructions and dont forget to leave a rating for me. Thanks!!!
http://www.coscojuvenile.com/instructions.asp

Remove straps and belt across the lap!