Dear Tirupurari,
Kindly visit the relevant ITO-TDS and ask for the details of mistakes/mismatches. I believe a part of this info is also available on TIN-NSDL website now (i am not sure of this part as I haven't done a TDS return correction in last one year).
Generally, the ITO shall provide you with the details of mistakes in print out/ soft copy. On basis of the same you may correct the TDS return. In case any mistake pointed by ITO has been wrongly pointed out or is on account of wrong judicial interpretation, you may submit written submissions in respect to the same.
Normally the above process should take care of the demand raised. However, if some portion of deman raised is on account of actual short deductions by the deductor, the same has to be paid off.
If you are not satisfied byt he final order passed by ITO-TDS, you have the option of filing appeal with CIT (Appeals) TDS