You installed the subclipse plugin in the Designer to manager your SVN repositories.
You added your repositories and checked the box to save the user / password when the credentials where asked.
So now you don't have to enter the logins again at each operation.
The problem is that now you want to change the credentials and do not find any menu or option to change it.
As explained in the sublipse documentation that you can find below, the solution is, if you are using the SVNKit client (default) to delete the following file:
The credentials of subclipse are stored in this file, so after deleting it and restarting the Designer, they will be asked again and a new file will be created.
Here is the link to the subclipse documentation:
Deleting this file will remove also the passwords for any additional plugins that is using it to store credentials