I've downloaded neuronal RNA seq data from GEO. The files are in .counts.txt format. Could I convert them to bigwig? If so, how?
No, count-level data doesn't have the base-level resolution that is required in order to be usefully visualised on a genome browser. You need a BAM file (or raw FASTQ files that can be used to generate a BAM file) in order to do that.
"No" isn't completely correct: it'd be possible to take gene counts and information about gene annotation boundaries to shoehorn them into a BedGraph / BigWig file, but whatever comes out the other end of that isn't going to be a good representation of reality.