how do i change dell service tag in bios

Why do you need to change it?  This is, in effect, your computer's Serial Number.  It cannot be changed unless you have received a replacement motherboard from Dell (or you are a certified Dell technician with access to the Asset utility for setting the Service Tag) - then you will have a one-time shot at setting it to the correct tag number in the BIOS.

If you have issues in this regard, simply contact Dell support. They are available 24 hours a day to assist.

i have changed motherboard and now some drivers are missing. laptop is out of warranty, dell will not help.

Where did you get the motherboard?  What is the part number?  If not from Dell, you likely will not be able to change the Service Tag at all.  If it is a different model of motherboard, that would explain why you are missing drivers.  You may need to re-activate Windows at some point - depending on the model of the board, it may even void your OEM Windows license.

i bought it on ebay as spares parts same model of laptop

Same model of laptop ... but what is the actual Dell model number of the motherboard?  Each model of computer can have different revisions of motherboards - in some cases, different hardware is employed (graphics, sound, network, etc.).  What devices are showing unknown?

thanks for your help, but am becoming very agitated now. I am not a tech but an ethusiast I just want to know if there is a way to change or alter the tag.  I have tried similar ways but haven't worked and will try for the sake

No, there's not.  

It must come from Dell with the Service Tag cleared, or you must be a licensed Dell technician with the proper Asset utility.

Well just incase some finds useful, I have found a tool that you can change sevice tag in bios called dstcd, you just need to copy file to cd and make it bootable. slot it in your drive, reboot machine and follow prompts. It has worked for my problem and i just google servive tag and it use the generic drives. My laptop now works great without any glitches. Thanks for your help all.

This question has been classified as abandoned and is being closed as part of the Cleanup Program.  See my comment at the end of the question for more details.