Planning to upgrade solr 6.x to 8.x. kindly provide steps to upgrade?

You should start reading the complete documentation provided to upgrade to Solr 8.x from pre 7.x version, before starting the update process.

When upgrading between major versions (for example, from a 7.x release to 8.x), a best practice is to always reindex your data.

When even skipping a major version, like in your case, an index migration is not supported and requires a reindexing of the complete data.