It will depend on the kind of magnet you're using. Most ordinary magnets (not the really really powerful ones) will lose some or all of their magnetism when they're heated above certain temperatures; and a lot of magnets become better magnets in cold temperatures. This may be an interesting science experiment - please get the help of a parent or another adult for it!

You will want some hot and cold water in different containers, thermometers, the same type of magnets of the same size, paperclips or other things that the magnets can pick up, and goggles and heat-resistant gloves.

You will want to heat and cool the magnets for the same amount of time, using the hot and cold water, and know the final temperatures of the magnets by measuring the temperatures of the water in which the magnets sit. Then, you can use the magnets to pick up paperclips - please use gloves or tongs to handle the magnets here! - and see if the number of paperclips picked up has changed from before heating/cooling.

For best results, heat and cool the magnets for at least 5 minutes, and record the number of paperclips before AND after heating/cooling. For heating, you may want to go to a temperature higher than 80 degrees Celsius.