Java 26.2
Attributes
Max Health
Base Set for @p
/attribute @p minecraft:max_health base set 40
Entity stats
Attributes are the underlying stats of every living entity. Java 1.21.2 dropped the generic. prefix from the attribute ids, so a command that works on one version silently targets nothing on the other. Pick the version and the generator writes an id your world accepts.
Java 26.2
Base Set for @p
/attribute @p minecraft:max_health base set 40