If you have customized the plugin slightly, e.g. changed css a little or made some slight code change, you can update BUT before that please make backup to save the changes you have. After the update was done, transfer the changes to new version.
If you have customized the plugin greatly, i.e. added new features which can conflict with new code of the released version, we wouldn’t recommend to update the plugin. The only way to add new features/fixes to new version is to repeat customization process on the new version.
However you can still use the new version on multiple other websites.