Not sure if that is your situation, but FYI if your account is in suspended or expired status, you can't change plan by yourself, you have to contact the mods...
@ahmetkokulu, it is not you. The self serve portal is a terrible piece of software. Try using Chrome and incognito mode. This seems to work well for most people. If required, you can get a moderator to make the chang for you. The moderator team can be reached via private message using this link. Refer to the following link for information on contacting the moderator team.
