As far as I know there are two ways to do it: Edit the text or delete the adopt page and replace it. Let me see if I can find the code I used to change the code on my site.
Deleting:
Once you are on your admin.php page replace admin.php in the link with
admin.php?set=settings&do=links&more=delete&evenmore=2
That will delete the adopts page. Then you add a link in the navigation bar linking to your new page.
Edit: There is a way to simply edit the page, but I'm not 100% sure how. I like making a new page because instead of just having an adopt pages you can have multiple pages. Obviously, personal taste comes into account.
According to Vestri on MA,
In the admin CP, make a new page with the URL adoptpagetext then in the content of it, write what you want it to show up as on the page and voila! Done~