The second parameter in the replaceAll function is a regular expression, and backslash is a special character in regular expressions, it must be escaped, that is write twice:
replaceAll($Account/FullName, '\\', '\\')
To replace those characters, you can set a Change object action inside your microflow and then use replaceall like so:
For that example I replaced the attribute FullName from the Account entity, you can change that part to use the attribute you want to replace:
replaceAll($Account/FullName, '\', '\\')
I hope that helps you, and if it does please mark the question as solved, best regards!