The easiest way to remove a chainring is usually to take the right-hand crank arm off the bike first. Chain rings are mounted using chain-ring bolts that clamp the ring to the 'spider' arms on the crank.
The outside part of the chainring bolt usually has a hexagonal socket which takes a hexagonal key. The inside may take a special key with two teeth or a hexagonal key. If you don’t have the key you might be able to do the job with a big screwdriver, but take care. Sometimes the inner ring of a three ring system is bolted directly into holes on the crank spider. These bolts may have a star-shaped Torx socket that needs a specific tool.
Hold the inside bolt still and turn the outside bolt anti-clockwise to get it moving. Run around the chainring and move all the bolts before dismantling any.
Disassemble the chainring
If the chainring is not symmetrical – sometimes the teeth are slightly off-set to the left or the right – note which way round it faces. Note the exact position of any washers or spacers, unscrew each bolt in turn and place all small parts in a tray. If the chainring has specially formed teeth to aid shifting take careful note of any markings that indicate the exact position of the ring.