How to downsample some of the samples in RNA-seq data?
You have a few options:


Downsample the fastq files and rerun the entire analysis. You can do this with seqtk sample.
Downsample the BAM files, which you can do with samtools view -s.
Divide all of the counts in the counts files by some factor and round that to an integer.


I personally prefer option 2, since it's quick and doesn't usually have any side-effects unless you use two-pass alignment or did the quantification with something like salmon. Option 3 is always tempting, but I have to say I've never been convinced that this consistently produces similar enough results to options 1 or 2.