To substitute borax (decahydrate form) for the soda ash when using the Taylor base demand test use twice the amount of borax by weight then the amount of soda ash called for.
The decahydrate form of borax is commonly sold as 20 Mule Team Borax.

HOWEVER, this is only for raising pH when the TA is good and you don't want it to rise any higher and is not part of the TA lowering process, which involves dropping the pH to a safe level (7.0), aerating to bring the pH back up and repeating this process. When you add acid to lower TA you will also lower the pH by converting bicarbonate into carbonic acid (operative word here is acid). Carbonic acid, for our purposes is just carbon dioxide dissolved in the water (think club soda or seltzer). If we areate the water to drive out the CO2 (think shaking the bottle of club soda to make it go flat) we have lowered the amount of carbonic acid (operative word is acid) and the pH goes up. However, the carbonic acid is not converted back into bicarbonate but has been removed from the water so the TA does not climb. Once the pH has risen you can add more acid to lower the TA and pH again and repeat this process until the TA is where you want it.