Go to Fundraising > Donation Pages. To locate the page that you would like to edit you type the name of the page in the "search" field or you can sort the Donate Page list by name.

After you locate the page that you want to edit click the "Edit Page" button.

If you want to edit General Information such as name this info can be located on Step 1. 

Design elements such as donate page copy and colors can be found on Step 2. 

On "Step 3" you can edit information displayed on the "Thank You" page. This info includes Thank You graphic and message. 

At any time after making an edit simply click the "Save Draft" or "Finish" button.