I would like to change some settings in BIOS.. How shall I enter the BIOS after Ubuntu has fully started?
BTW: I have been in the middle of working on something under Ubuntu for quite a few days. I don't want to restart it.
You can't "enter" your BIOS without rebooting. However, BIOS settings can be changed during runtime. It very much depends on the BIOS you are running. Have a look here: http://stackoverflow.com/questions/6259124/apis-for-querying-and-setting-bios-properties for more information.
Also, a lot of what you might want to change is exported to /proc and/or /sys and can be changed rather easily. A lot of all of this depends on the vendor of your motherboard. For Dell (just used as an example), there is libsmbios which already gives you the possibility of editing some bios settings.