I think that you are asking about recharacterizing a Roth IRA contribution made for 2018 to be a traditional IRA contribution instead.
Whether or not a traditional IRA contribution instead of a Roth IRA contribution will reduce your 2018 tax liability will depend on whether or not the traditional IRA contribution is deductible. See the following IRS web page to determine if a traditional IRA contribution will be deductible: https://www.irs.gov/retirement-plans/ira-deduction-limits
Note that making a deductible traditional IRA contribution only defers taxation of the contribution. The money and any earnings on that money will eventually be includible in income when distributed from the traditional IRA. Contributions to a Roth IRA will not be includible in income when distributed and the earnings thereon will not be includible in income upon distribution provided the requirements for a qualified Roth IRA distribution are met. For younger contributors, the long-term tax benefits of a Roth IRA contribution will usually outweigh the short-term benefit of a deduction for a traditional IRA contribution. Of course if the Roth IRA contribution is an excess contribution, to avoid penalties on the excess contribution you'll have to either recharacterize the contribution to be a traditional IRA contribution (deductible or not) or obtain a return of the Roth IRA contribution.