How do u replace a starter string on a troy bilt TB90BC trimmer?

Hi,
If this one has the "Easy Start" or whatever feature, then it is an unitized assembly. It is not worth trying in a different way. You will probably "pull your hair out" and still fail.
Manual should be at OwnerIQ: Product user manuals, help, forums tips and tutorials. (http://owneriq.net/) just follow the menu progression on the left.
Peace,
Clarke

I don't need the operators manual I need a repair manual all that site gives is operator. Please someone help me I'm getting over grown with weeds there already 4 foot tall

Hi,
There is no service manual freely available, that I know of. I learned about lots of brands by the parts breakdowns. You can get to them through various dealers or distributors that allow you to go to the ARI site through their site. An example:
http://www.milacalawn.com/parts.asp
Peace,
Clarke

Well thanks but its been a waste of time ill just take it to a shop since no one can explain how to fix it on here