That depends on depth of changes, fixing typo requires no discussion, for massive changes like deprecating entire highway= key even proposal is not good enough.
For documenting de facto tag/value - see Creating a page describing key or value - OpenStreetMap Wiki and note Any tags you like - OpenStreetMap Wiki
For redefining existing tags (and in some way also for adding new values to keys where there is closed set of values) - see Deprecation process - OpenStreetMap Wiki and Deprecated features - OpenStreetMap Wiki
See also https://wiki.openstreetmap.org/wiki/Proposal_process
ask wider community here and/or tagging mailing list? In case of no opposition you are fine to proceed without proposal based on such consensus, in case of serious opposition I would consider proposal process.
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