Remove the old file (see https://help.researchgate.net/hc/en-us/articles/14293099743121-How-to-make-content-private-or-remove-it for instructions), then upload the new version.

To replace an old version by a new one, you may just remove the old version; then upload the new one. However, to keep priority of your publication over other authors' publications, you may want to keep the old version as published (especially if it is published in an academic journal or conference).. and upload the new version unless there is something in the old version that needs to be corrected.​