You cannot fix or undo the lack of withholding for several months. It is what it is. Going forward, change your withholding with the employer and have them take out federal tax. The results of your 2020 tax return will simply reflect the amount of income you had and the amount that was (or was not) withheld.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
