From the help: 1. Open Gmail. 2. In the top right, click Settings and then See all settings. 3. Scroll down to the "Button labels" section. 4. Select Text.

From the help: 1. Open Gmail. 2. In the top right, click Settings and then See all settings. 3. Scroll down to the "Button labels" section. 4. Select Text. 5. At the bottom of the page, click Save changes. The text also has the benefit of making the buttons much bigger / easier to click.