If you are using OptimizeMember and you want to change some of the strings of text used on the page elements such as the login form, the order form etc, you can do this using a translation plugin such as "Loco Translate"
Note: Please note that some elements have built in options so always check first that we do not have customization options within the element options
If you need to change a text string, install the Loco Translate plugin then go to Loco Translate > Manage Translations
Now look for OptimizeMember on the list and click "+ New Language" under the Translations (PO) column
If your site/server is in English, select English from the dropdown and click "Start Translating" (en_US is the standard for WordPress unless you have installed a different language locale)
Now you can search for your text string you want to modify and add a translation:
Save your changes and your new strings should be live as long as you selected the correct language for your translation according to your server settings