When you made the January 2019 HSA contribution, you needed to have told the HSA custodian at that time that the contribution was for 2018. The default for the custodian is to assume that the contribution is for the year in which the contribution is made.
You can try at this point to call the HSA custodian and claim that the January 2019 contribution was actually for 2018, but at this late date (HACKITOFF is right that this should have been done by April 2019), the custodian is under no obligation to oblige you. Note that the custodian files reports with the IRS, so this is not just a matter of you claiming that that contribution was for 2018.
If this does not work out (and I would not be surprised if it doesn't), then note that you can pay for qualified medical expenses years after the expense was made, so long as the medical expense was incurred after your HSA first existed.
That is, if you had more expenses in 2018 than you could pay with your HSA at the time, then later - say in 2020 - when you have additional HSA funds, you can contact the HSA custodian to be reimbursed for expenses that you had made in the past with after-tax funds. The IRS places no limit on the amount of time that might pass to do this. Of course, any contribution to your HSA in 2019 or 2020 or later years creates a tax deduction for you, so even though you are repaying yourself, you will have the tax benefit that you would not have had otherwise.
What you need to do is to keep proper records so that if the IRS ever audits you for HSA contributions and distributions, you will be able to show why the reimbursement was justified.
When you made the January 2019 HSA contribution, you needed to tell the HSA custodian at the time of the contribution that the contribution was for tax year 2018; otherwise, the custodian would assume that the contribution was for tax year 2019.
Now, as Hackitoff says, it is too late to contribute to your HSA for tax year 2018, because that needed to be done by the due date of the return (April 15, 2019 or if you filed for an extension, October 15, 2019).