Going back from the managed schema to a classic schema.xml you edit yourself is one element in solrconfig.xml, plus a reload.
01 · The change
In solrconfig.xml, find the schemaFactory definition. If there is one, remove it and put this in its place:
<schemaFactory class="ClassicIndexSchemaFactory"/>
If there is no schemaFactory at all, add the line anywhere above the request handler definitions.
02 · Then
Reload the index
From the Tools menu of your index control panel. Solr reads schema.xml from that point on.
Only one factory may be present
If both the classic and the managed factory are left in the file, the core will not start.
Your fields live in schema.xml again
Edit them in the config files editor. The Schema API stops being able to write, which is the point of classic mode.
Going the other way, to managed mode, is described in move from schema.xml to managed-schema.