First use a hot air gun, or hair dryer, to warm the problem part of the tile.
Then carefully lift it slightly beyond the original problem.
Now, clean off the floor and the tile from the years of dust that has accumulated under this slightly curled tile.
Put contact cement under the corner.
Push the tile into place and then pull it back up. This will spread contact cement evenly on both sides of the problem. You may need to re-heat the vinyl to keep it warm during this process.
When the glue is tacky, press the corner of the tile into place and put weights on it to keep it down until it is dry.
Heat can reactivate contact cement and softens the vinyl as well so that your can force it back down if it starts to curl again.