In order for your Thornmore knife to provide you with years worth of service you will need to maintain it.
You should keep your knife clean and gently wipe the blade with a light oil to ensure that it is kept in good working order.
Your Thornmore knife should be kept sharp – a dull blade is a dangerous blade. There are many different sharpening devices and techniques out there; there is no definitive right way to sharpen a blade. Practice (with a cheap knife to start with) and gain experience. If you do not have the necessary equipment or are not confident sharpening your knife then there are companies who will do this for you (research the company first and try to find honest reviews about them online or from friends).
Disassembling your Thornmore knife could lead to voiding the warranty and result in injury if it is not reassembled correctly.