you seem to be using the OpenSource Version of GVM. No official upgrade path is given for such installations apart from the documentation enclosed with the source.
That being said, you should elaborate more on the steps you took and the problems encountered. Then you may have a chance, that you get an answer.
For official support you will have to by the commercial version.
I did an upgrade from 20.8.0 to 20.8.1 a few days ago on ubuntu server 20.04 LTS.
After trying to find a documentation for this upgrade, in vain, I followed these steps:
rebuild from new sources gvm-lib, gvmd, openvas and gsa in new outpout folders (using -DCMAKE_INSTALL_PREFIX)
make symbolic links for the next upgrade (ex.ln -s /opt/gvm-libs-20.8.1 /opt/gvm-libs)
reference these symbolic links in /etc/ld.so.conf.d/gvm.conf and /etc/ld.so.conf.d/openvas.conf
4.use these paths in my startup scripts
5.Modify scanner to point the good sock gvmd --modify-scanner=UUID_SCANNER --scanner-type=“OpenVAS” --scanner-host=/opt/openvas/var/run/ospd.sock
After restart, all the components were in version 20.8.1.