You will have to modify the qa-lang-*.php files in order to do so. Follow the steps in http://www.question2answer.org/qa/41606/how-can-replace-the-word-question-in-whole-website-with-post?show=41610#a41610 . For 1.7 the solution is the same but you'll have to copy files from qa-include/lang/qa-lang-*.php instead of qa-include/qa-lang-*.php
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July 15, 2020: Q2A 1.8.5 released!