Hi Patricia,
If you're brazilian too we can help each other in portuguese at the private messages 
So, to export your table.qza or any other file from qiime2 to R you can follow the qiime2R tutorial, it will give you the steps to introduce your files in R, here are the github from qiime2R too.
Once you're with your qiime files in R you can follow the tutorial from Decontam. I recommend to look to the number of negative controls that you have in your dataset, because Decontam works well with a certain number of this type of samples.
Another option is to use microDecon, it worked well with my dataset that just have two negative controls (one from extraction and other from PCR). If you need Here are the microDecon tutorial.