I have not filed my return yet but need to switch from single to head of household and can't find where to do it.

Have you entered a child under My Info?  Then if you qualify it will change it to Head of Household.

Who is a qualifying person for Head of Household

https://ttlc.intuit.com/community/family/help/what-is-a-qualifying-person-for-head-of-household/00/26624

See FAQ on Head of Household

https://ttlc.intuit.com/community/family/help/do-i-qualify-for-head-of-household/00/25539

If you need to change your filing status from Single to Head of Household, in TurboTax Online, open your return and select the Personal Info section. This is where you enter all of the information the program uses to determine your filing status. 

Head of Household filing status typically allows parents or adults with qualifying dependents who provide over half the cost of keeping up a home for a qualifying person to claim a higher standard deduction and be taxed at lower tax rates than single taxpayers or those who are married filing separately.

See this TurboTax tax tips article for more information regarding head of household status.