Below are some step by step instructions which should solve your issue.

1. Open the repository on GitHub and navigate to the Issues tab.
2. Click on the issue you want to add a label to.
3. On the right-hand side of the issue, you should see a section called "Labels". If there are already labels assigned to the issue, they will be displayed here.
4. To add a new label, click on the "Labels" dropdown and start typing the name of the label you want to add.
5. If the label already exists, it will appear in the dropdown list and you can select it. If it doesn't exist, you can create a new label by typing its name and pressing Enter.
6. Once you've selected the label(s) you want to add, click on the "Submit new label" button.