If you have a credit note in your accounting system, it will be offset against the supplier who issued it.
To reduce the bill amount in Comma, you will need to apply the credit note against particular invoices. The way to do this varies by accounting software:
1️⃣ Open your credit note in Xero
You can find this by a search.
2️⃣ Select credit note options, then allocate credit
The amount available will take into account any credit which was already allocated.
3️⃣ Enter the amount to credit, and select allocate credit
Repeat this process for each invoice you want to apply credit to. You can allocate the full amount, or simply part of the total.