Hi Rick,
yes you can reset the thank you page by editing your form in the landing.
Steps:
1. Select the form
2. Click on the form >> Click on the setting icon >> Click on Edit
3. In the "Follow-up Type" you have two options to direct the person:
a. Landing Page (existing insided marketo)
b. External URL (external web page)
4. Click Insert.
-Ramesh