Using dbms_workload_repository.modify_snapshot_settings one can modify retention, interval and topnsql.

-- get the dbid which is needed to passs to dbms_workload_repository.modify_snapshot_settings
SQL> select dbid from v$database;

DBID
----------
1992878807

-- retention=>value in minutes so (45 days * 24 (hours per day) * 60 minutes per hour = 64800), max value can be set by passing a value of 0 which means forever
-- internal=>60min (snap at this interval), a value of 0 will turn off AWR
-- topnsql - top N sql size, specify value of NULL will keep the current setting
SQL> exec dbms_workload_repository.modify_snapshot_settings(retention=>64800, interval=>60, topnsql=>100, dbid=>1992878807);

PL/SQL procedure successfully completed.

-- shows retention and interval after it was modified
SQL> select extract( day from snap_interval) *24*60+extract( hour from snap_interval) *60+extract( minute from snap_interval ) snapshot_interval,
extract( day from retention) *24*60+extract( hour from retention) *60+extract( minute from retention ) retention_interval,
topnsql
from dba_hist_wr_control;

Snapshot Interval Retention Interval topnsql
----------------- ------------------ ----------
60 64800 100

-- Change snapshot interval to 30mins
SQL> EXEC dbms_workload_repository.modify_snapshot_settings(interval=>30);
PL/SQL procedure successfully completed.