The text on your partner portal's sign-up screen comes from the description of a partner group. To change it, edit that group's description.
Go to Program settings › Partner groups and open the group.
Update the Description.
Click Save changes.
Which description partners see
Your main portal link shows the description of your default group.
A group's own sign-up link shows that group's description.
So if you have several groups, you can give each one its own message, for example the commission rate that group earns. See Partner groups.