How do I change who I filed with the previous year? I forgot that I filed with this service instead who I usually file with (H&R block).

[VolvoGirl]

You don't need to indicate that anywhere.  Who are you filing 2018 with?  You will need to enter your 2027 AGI to efile2018.

if you used Turbo Tax for 2017.....

How to find last year's AGI 

https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/00/25947

[Critter]

If you are just starting the return and want to start over click on the CLEAR & START OVER option under the TOOLS section on the left ... then you can choose who you used last year. If you used the online version then you have to be logged into the same account for the prior year info to transfer over.