Thank you very much.
That icon is a little bit tricky as it's a vectorgraphic SVG image (to support 4k devices), and it's currently not changeable on the back end. It's been requested that I add this feature before, so it's definitely going to be available in the upcoming version.
In the meantime I can explain briefly where you find the code that's including the SVG element so you can change it:
If you open up the wp-content/plugins/ajax-search-pro/includes/views/asp.shortcode.data.php file and check lines 5-13 you should see an SVG html element between opening and closing tags.
Basically that's the icon in a vectorgraphic format.
You can replace that whole code with an image element if you have your own magnifier icon created,
Or you can browse for the built in magnifier icons in the wp-content/plugins/ajax-search-pro/img/svg/magnifiers/ folder, where you should see like 7 different .svg files.
If you double-click it should be opened in IE or your default browser to show a huge magnifier.
If you like one of them, then open that svg file with a text editor and copy all of it's content.
Now go back to the php file previously opened and replace lines 5-13 with this new code.
It should be very similar in structure, only some number differences. After refreshing your browser you should be seeing the selected icon instead of the old one.
Sorry if it's a little messy, I honestly forgot to add an option for that icon in the current version.Best,
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