How do I file an amendment for refund of a reimbursement paid in 2018, for a relocation I did in 2017 (which should not be taxable). Can I do this through Turbo tax?

you can not amend the 2017 return since the repayment occurred in 2018

if $3,000 or less not deductible in 2018 (it was a 2% agi item which was eliminated) 

if over $3,000 claim as a credit on 2018 return.

recompute your 2017 tax without the reimbursement

the difference in tax between the original return and the recomputed return is claim as a credit on schedule 5 line 74  check box d and write IRC 1341

Thanks for your answer. It is over $3000. The problem in this case is that my employer already removed the tax when paying me the reimbursement, so I got it paid out as non-taxable income (but only about 2/3 of what I should have got). Is the employer obliged to pay tax on the reimbursement or should they simply have paid me the whole amount without removing tax?