Usually, invoices are uploaded on behalf of your Legal Service Providers from within Engage > Legal service providers > Invoices. Read how here.
However, there is a more convenient facility whereby you can upload invoices within a matter.
Select the Invoices tab within the matter. This is typically where you would see all the invoices that have been uploaded against the matter.
Click the + Add Invoices button in the top right.
A modal will pop up granting you two options:
Upload invoice and post - the invoice will appear against the matter straight away. Or,
Add to an existing invoice batch - you will be directed to Invoice batches where you can upload the invoice to the desired batch.
If you select Upload and post the file picker will be displayed. Simply select the desired file(s).
Enter the invoice details into the left-hand panel and hit Post batch.
The invoice will instantly appear in the matter awaiting approval.