It depends what you want to learn, just improving your Japanese level overall is a hard thing to do. You have to break it down into reading comprehension, writing, listening and speaking. For reading comprehension and writing I’d say get an intermediate book like Tobira but if you don’t want to do that pick up a relatively easy 小説 and work through that. Note down grammar and vocab you don’t know and make an effort to properly understand everything. It’ll be a lot to get through at first but you’ll speed up quickly.

I like to do listening passively as well as actively. There’s plenty of listening exercises on YouTube and better ones that come with textbooks. There’s also a really good podcast for around your level called にほんごザッツ談 that is great for passive listening. Music and TV even with the subtitles on definitely helps.

For speaking there really only is speaking to native people. You can get teachers online to do video calls with or use the app HelloTalk for free to meet people. I did this for about a year before I came to Japan and it really helps. It gets you used to talking on the phone, texting in a normal way. And some people will want to do proper lesson style calls where you can improve speaking a lot.