Note: You can only upload TTF (True Type Font) formats. 

Step 1:
Select Brand Kit ↓
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Step 2:
Select Add Font ↓

Step 3:
Select Upload and choose the font you would like to upload. → You may rename your font → Click Add

Step 1: Add some text to the canvas by clicking the text icon on the toolbar.

Step 2: Select the text on the canvas and navigate to the Edit tab.

Step 3: Click the font name to access the list of fonts, then click the Upload font to brand kit button on the bottom.

Step 4: A pop-up window will appear. Select Upload and choose the font you would like to upload. → You may rename your font → Click Add