Go to STS-> Help -> Install new Software -> add the below URL
http://dist.springsource.org/snapshot/GRECLIPSE/e4.5/
try with : http://dist.springsource.org/snapshot/GRECLIPSE/e4.5/ instead of https://dist.springsource.org/snapshot/GRECLIPSE/e4.5/
Ref for Use one of the following update sites depending on your version of Eclipse: https://github.com/groovy/groovy-eclipse/wiki/3.5.0-Release-Notes
https is giving the error like "Unable to read repository at https://dist.springsource.org/snapshot/GRECLIPSE/e4.5/content.xml.
Unable to read repository at https://dist.springsource.org/snapshot/GRECLIPSE/e4.5/content.xml.
PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target"