Need to update new column created on table with data processed from same table

update UC_OFCOM_HIST_TRANS_LOG set LOAD_DATE_TIME = (select to_char(to_date(substr(a.file_name,instr(a.file_name , '',1,4)+1,length(a.file_name)-instr(a.file_name,'',1,4)-4),'YYYYMMDDHH24MiSS'),'DD/MM/YYYY HH24:Mi:SS) from UC_OFCOM_HIST_TRANS_LOG);

getting an error: ORA-01427 Single-row subquery returns more than one row