yq/pkg/yqlib/doc/headers/assign-update.md

Assign (Update)

This operator is used to update node values. It can be used in either the:

plain form: =

Which will assign the LHS node values to the RHS node values. The RHS expression is run against the matching nodes in the pipeline.

relative form: |=

This will do a similar thing to the plain form, however, the RHS expression is run against the LHS nodes. This is useful for updating values based on old values, e.g. increment.