To change your username/password for Justgiving you can easily overtype the details within the boxes by clicking into and amending each one in turn.
It is important not to use any autofill options that your website browser is presenting as these may not contain the correct credentials
If your browser is overruling everything and you are not able to free-type into the fields then, you could disable the option within your website browser.
Click into the Username field, and then delete the user name. Freetype the correct user name.
Next, click into the password field, and then delete the user name. Freetype the correct password.
Once the details have been entered, click on the Save button.
Next, click the Test Settings button to make sure that the new credentials are all working. If the test passes, super your integration is good to go.
If the Test fails, a red banner will appear which provides details from JustGiving as to the failure reason. Correct as required, save and test again.
For general error messages see these articles: