If you are not using an image to display a button, you can customize the text on the button in the Localization you are using for this page. To find out if you are using an image for the button, edit the Style you are using for this page. On the Define Style Settings screen, for the element Forms/Details, click Buttons. In the element type, Image Replacements you can check if you have enabled image for Submit/Update button.
To customize the text on the button edit the Localization you are using for the page and navigate to Define Localization Setting screen. Look for Forms/Details and choose element Buttons. Now enter any custom text in the Custom Text area. Click Save or click on another row to save the setting.
Please note the changes you make to a style or localization will affect all the pages that are using them. If you would like to make these changes only in one DataPage, create a copy of the Style/Localization, make the changes and apply the copy one to your page.