Hi century,
In order to upgrade existing 1.0.3 you just need to check "Check for updates" on the DNP installer. Also please note that update perform against all of installed components (Portal, ES, Server) not a particular one.
Steps to upgrade 1.0.3 to 1.5.1:
1. Backup all existing data (files and databases);
2. Download and run the latest DNP installer;
3. Tick "Check for updates" box and proceed with the upgrade wizard steps;
4. If any errors were appeared during installation - restore all of your files and databases back and email to our support with error log attached;
5. If update process is succeed then open portal, try to login and check whether all is ok.
6. Repeat 1 - 5 steps once until you reach desired version;
There are no any localization penalties upgrading 1.0.3 to 1.5.1, but there are some things that should be considered:
1. If you need to localize some files for your specific needs, create a copy of these files and add your culture code for them. For example: AuditLog.ascx.resx -> AuditLog.ascx.de-DE.resx, it assumes that de-DE file contains Deutch locale customizations.
2. If you had already original files localized then just add your locale code. For example: AuditLog.ascx.resx -> AuditLog.ascx.de-DE.resx, hence your locale customizations never won't be discarded with current one and future updates.
3. Please note that shared resource file (SharedResources.resx) were renamed to SharedResources.ascx.resx, thus you allowed to use DNN resource editor;