To add files to existing beats, or replace existing files on your beats head over to Dashboard > Music & Media Beats > Upload
Click the Replace option
Drop your files onto the uploader, or click inside the dashed box and select your files
Once your files have uploaded, you will be able to choose which beats you want to link the files to.
Select the relevant beat for each file and click Save
Please note - if you are replacing any files you will also need to replace the tagged file, as this doesn't happen automatically. To do this, simply select the beat and run the Autotag again (or upload your own new tagged version)