Can we upgrade SH version of 6.1.2 to 6.5 while we are planning to continue other components (Indexer,Deployer) still on 6.1.2 versions?
I am looking at documentation, but didn't find a solution for this, can someone please suggest us?
yes you can do it following the usual upgrade procedure, I did it (indexers 6.1.5 SH 6.5.0).
You only have to install the same Splunk version in the same server roles if you have a cluster.
Yes, you can just upgrade the search head(s). The installation procedure is the same.