Need help upskilling

So I'm currently working as a c++ developer and am involved in development for a cloud agent since past 3.5 years, this is all the work experience i have. Working with C++ and qt framework. I like what I do right now as I am heavily involved in designing and developing the solutions. I'm thinking of upskilling but am confused on what to learn so that my current experience counts and will also help me switch to something better. Looking forward to all the suggestions!