Usually, the easiest way to improve a laptop's overall performance is to replace a hard drive with an SSD, which is much faster.
If you already have an SSD, you can still benefit from upgrading – newer or more premium SSD options for your laptop might exist. If your SSD is close to full, this will slow down performance. Storage drives work faster the more free space they have, so a bigger SSD might come with the benefit of faster speeds… until you fill it up, too.
Usually, a laptop will only have one or two RAM slots available. Sometimes it has a spare, empty RAM slot. If this is the case, you can potentially just buy new RAM and not remove the old, as long as the new RAM is identical.
Make sure you check online and buy the right RAM for your model. You may be limited in the capacity of the RAM module you can install, and different types of RAM require different port shapes.
Laptop batteries wear out quickly. Replacing yours with a fresh one can give you much longer between charges, even though it'll probably be the same model battery as your old one. It won't boost performance, however, unless your current laptop spends a lot of time in power-saving mode.