can re installation plesk core package can affect existing working plesk or not ?
Just download this package for your OS from
http://autoinstall.plesk.com/PSA_12.5.30/ and install it with OS package manager (rpm or dpkg) over existing package. It will not affect your Plesk installation. For example, for CentOs 6 32-bit:
[root@ppu12-5 usr]# plesk version
Product version: 12.5.30 Update #12
Update date: 2015/11/18 04:41
Build date: 2015/10/23 17:00
OS version: CentOS 6.7
Revision: 344620
Architecture: 32-bit
Wrapper version: 1.2
[root@ppu12-5 usr]# wget
http://autoinstall.plesk.com/PSA_12...core-12.5.30-cos6.build1205150826.19.i686.rpm
--2015-11-18 17:08:07--
http://autoinstall.plesk.com/PSA_12...core-12.5.30-cos6.build1205150826.19.i686.rpm
Resolving autoinstall.plesk.com... 10.52.1.73
Connecting to autoinstall.plesk.com|10.52.1.73|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 27960108 (27M) [application/x-redhat-package-manager]
Saving to: “plesk-core-12.5.30-cos6.build1205150826.19.i686.rpm”
100%[=================================================================================>] 27,960,108 76.5M/s in 0.3s
2015-11-18 17:08:08 (76.5 MB/s) - “plesk-core-12.5.30-cos6.build1205150826.19.i686.rpm” saved [27960108/27960108]
[root@ppu12-5 usr]# rpm -Uvh --force plesk-core-12.5.30-cos6.build1205150826.19.i686.rpm
Preparing... ########################################### [100%]
is running
Stopping sw_engine service... done
-- Warning: Skipping the data of table mysql.event. Specify the --events option explicitly.
===> Cumulative APS controller database (apsc) upgrade has been started.
===> Cumulative upgrade of APS controller database has been completed.
===> Cumulative Plesk database upgrade (revertable stage) has been started.
===> Preparing Plesk database upgrade (revertable stage).
===> Cumulative upgrade of Plesk database (revertable stage) has been completed.
1

lesk-core ########################################### [100%]
===> Cumulative APS controller upgrade (final stage) has been started.
===> Cumulative upgrade of APS controller (final stage) has been completed.
===> Cumulative Plesk upgrade (final stage) has been started.
===> Preparing Plesk upgrade (final stage).
===> Cumulative upgrade of Plesk (final stage) has been completed.
[root@ppu12-5 usr]#