In the Find box, put:
(\\texttt\{)(.*?)(\})
and replace:
**\2**
Explanation:
The Find box contains three sets of brackets: (\\texttt\{), (.*?) and (\}), this means it is trying to capture patterns that match these three statements in a row. The slashes in the first capture indicate that they are escapes, so we're looking for the literal \texttt{ but we need to escape the first slash and the curly brace by adding a slash before them. Similar for the third capturing brackets. The middle capture is what's most interesting: (.*?) will capture any symbol because of ., any number of times with *, and it will do so lazily, thanks to?. The laziness means that if a string matches the third capture, it will stop searching. This is to avoid it matching something like "\textt{foo} bar. Hello \texttt{World}" as single instance, which would be greedy. Lazily, this string would match the search regex twice as you'd expect.
The Replacement, **\2** says whenever we find something in the find box, replace it with ** followed by the second capture, \2 and then another `**.