Download to your server jweb* package :

root@oliver% cd /var/tmp
root@oliver% ls
install				preinstall_boot_loader.conf
jbundle-8.4R3.3-domestic.tgz	sampled.pkts
jinstall-8.4R3.3-olive.tgz	vi.recover
root@oliver% ls -al
total 442492
drwsrwxrwx   4 root  wheel        512 Dec 23 14:34 .
drwxr-xr-x  27 root  wheel        512 Dec 23 14:32 ..
drwxrwxrwx   2 root  wheel        512 Oct  8 19:57 install
-rw-r--r--   1 root  wheel  111641482 Oct  8 19:49 jbundle-8.4R3.3-domestic.tgz
-rw-r--r--   1 root  wheel  114782972 Oct  8 19:39 jinstall-8.4R3.3-olive.tgz
-rw-r--r--   1 root  wheel         67 Oct  8 19:41 preinstall_boot_loader.conf
-rw-r-----   1 root  wheel       2314 Dec 23 14:34 sampled.pkts
drwxrwxrwt   2 root  wheel        512 May 26  2004 vi.recover
root@oliver% fetch http://192.168.0.1/jweb-8.4R4.2-signed.tgz
jweb-8.4R4.2-signed.tgz                       100% of 3870 kB   19 kBps 00m00s

Then go to CLI and :

root@oliver> request system software add jweb-8.4R4.2-signed.tgz

Installing package '/var/tmp/jweb-8.4R4.2-signed.tgz' ...
Verified jweb-8.4R4.2.tgz signed by PackageProduction_8_4_0
Adding jweb...
Available space: 412224 require: 6066
Mounted jweb package on /dev/vn7...
Verified manifest signed by PackageProduction_8_4_0
Executing /packages/mnt/jweb-8.4R4.2/mount.post..
Reloading /config/juniper.conf.gz ...
Activating /config/juniper.conf.gz ...
mgd: commit complete
Restarting mgd ...
Saving package file in /var/sw/pkg/jweb-8.4R4.2-signed.tgz ...
Saving state for rollback ...

WARNING: cli has been replaced by an updated version:
CLI release 8.4R3.3 built by builder on 2008-01-18 07:39:53 UTC
Restart cli using the new version ? [yes,no] (yes) 

Restarting cli ...

root@oliver> 

root@oliver> configure
Entering configuration mode
[edit]
root@oliver# commit ?
Possible completions:
  <[Enter]>            Execute this command
  and-quit             Quit configuration mode if commit succeeds
  at                   Time at which to activate configuration changes
  check                Check correctness of syntax; do not apply changes
  comment              Message to write to commit log
  confirmed            Automatically rollback if not confirmed
  |                    Pipe through a command
[edit]
root@oliver# commit and-quit

Related posts:

1. First steps configuring Juniper Routers
2. NetBSD adding users
3. Enable SSH