[SunHELP] sed-help

#!/dear/scripter

I need to modify thousands of db files in my DNS server.
I need to change 86400 in line
'        86400           ; Minimum timer - 1 day' 
to be  60 
and 43200 in lin
'	43200           ; Refresh timer - 12 hours'
to be 3600

in every file of db file directory.

I've try with sed -e 's/86400/30/' db.file
but the modified file only show on stdout not directly modify the file's
content.

$TTL   86400
@ IN SOA ns1.xxx.com. hostmaster.xxx.com. (
        2001100201      ; Serial number
        43200           ; Refresh timer - 12 hours
        3600            ; Retry timer - 1 hour
        604800          ; Expire timer - 1 week
        86400           ; Minimum timer - 1 day

Pleases suggest,
Patrick.