I filed single with a dependent for 2018.
Sure. Be sure that you qualify. See this article:
https://turbotax.intuit.com/tax-tips/family/guide-to-filing-taxes-as-head-of-household/amp/L4Nx6DYu9
Yes, if you are eligible to file as Head of Household:
You have 3 years from the date you filed your return (or 2 years after you paid the tax due, whichever is later) to file an amended return.
Please click here for how to amend a return you already filed
Link for Head Of Household requirements