Update table1
set Update_time = (case when value_a < 0.1 and Update_time is null then sysdate
when value_a > 0.1 and Update_time is not null then null
else Update_time end);
Change sysdate to your desired value.
EDIT:
Include Edit in the merge statement. See the below query (not tested with the real data) In this way we do not run the update on entire table.
Merge into table1 t1
using table1_staging t1s
on t1.name = t1s.name
when matched then
update t1.value_a = t1s.value_a,
t1.Update_time = (case when t1s.value_a < 0.1 and t1.Update_time is null then sysdate
when t1s.value_a > 0.1 and t1.Update_time is not null then null
else t1.Update_time end)
when not matched then
INSERT (name, values_a)
VALUES (t1s.name, t1s.values_a);