It's hard to see enough detail to diagnose, but this resembles thrips damage, and there might be an overlapping leaf spot infection present as well. (Sometimes heavy insect or mite damage can resemble infections.)

Do you happen to mist the leaves with water? If so, discontinue or improve air circulation if growing in a high-humidity environment. Wet leaf surfaces can be easier for plant disease spores to infect, so the faster they dry off, the less vulnerable they will be (to a point).

For now, you can clip off any heavily-damaged leaves like this one. Few (or no) fungicides are labeled for use indoors if this is a leaf spot, though if growing the plant outdoors for the summer, it might be an option if insect or mite damage can be ruled-out as the cause of browning. A few insecticides labeled for indoor use will work on thrips, but sometimes thrips are resistant to certain insecticides, so treatment in that case may be a process of trial-and-error. If you think you see thrips or mites and would like confirmation, feel free to share close-up photos.

Miri