There is 2 ways to change this text
1.Localization
Install localization plugin such as Loco
Under the Loco > Plugins > Groovy Menu you will see the translation files that are related to our plugin update checker module. The files are marked as Additional files found in the section. There are many different languages. But the phrases he needs are not there.
You need to create your own translation file
2. Custom search placeholder
Create a menu block and add the code
<p>START TYPING AND PRESS ENTER TO SEARCH</p>
<form action="/" method="get" class="gm-search-wrapper-form">
<div class="gm-form-group">
<input type="text" name="s" class="gm-search__input">
<button type="submit" class="gm-search-btn">
<i class="fa fa-search"></i>
</button>
</div>
</form>