11 May 2018
You can file an application with you Judrisctional Assessing Officer.
There is also an option available online on TRACES to change the relevant Assessment Year of a challan
13 May 2018
As per my view and experience, there is no issue for plus or minus of ONE Assessment Year in challans. That is, challan having ASST YEAR as 2018-19 can be used for AY 2017-18 and AY 2019-20 period ETDS filings. If in your case, the selected ASST YEAR is more than this you may use the option in TRACES website, as told by CA Ankit.