This is the first time to replace toilet gasket.
A few days (or months) ago, I found there is a water leakage between the head and body of the toilet, and finally, I disassembled it and found the gasket has been quite out of date.
While researching in YouTube, I realized that there are two type of gasket. One is having two screw and bolts and a single gasket, and the other one is triangle gasket, which is mine.
Replacing of it is not difficult, but it was hard to unscrew the old bolts since lots of rust there.
To remove the rust easily, I found torching it for a several seconds which helps to melt the rust a little bit. By doing this, I could take out the old stuck bolts.
Here is the good DIY video for toilet in YouTube.