Judging from the error messages, it sounds like Solr was previously installed. In that case, you can reinstall over the old installation by adding the -f flag to the installation command. E.g:

sudo bash ./install_solr_service.sh solr-5.5.1.tgz -f

It also seems that there is a stale pid file sitting around. Normally the existence of /var/solr/solr-8983.pid would imply that Solr was actively running, but as the process it refers to doesn't seem to exist, it might have shutdown uncleanly or crashed. You may need to remove that file before reinstalling. First check to see if Solr is really running. Run the command:

ps aux | grep solr

If it is running, you should see something like this in the output:

solr     32127  1.2 15.3 2499684 155976 ?      Sl   16:14   0:05 java -server -Xms512m -Xmx512m -XX:NewRatio=3 -XX:SurvivorRatio=4 -XX:TargetSurvivorRatio=90 -XX:MaxTenuringThreshold=8 -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -XX:ConcGCThreads=4 -XX:ParallelGCThreads=4 -XX:+CMSScavengeBeforeRemark -XX:PretenureSizeThreshold=64m -XX:+UseCMSInitiatingOccupancyOnly -XX:CMSInitiatingOccupancyFraction=50 -XX:CMSMaxAbortablePrecleanTime=6000 -XX:+CMSParallelRemarkEnabled -XX:+ParallelRefProcEnabled -verbose:gc -XX:+PrintHeapAtGC -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps -XX:+PrintTenuringDistribution -XX:+PrintGCApplicationStoppedTime -Xloggc:/var/solr/logs/solr_gc.log -Djetty.port=8983 -DSTOP.PORT=7983 -DSTOP.KEY=solrrocks -Duser.timezone=UTC -Djetty.home=/opt/solr/server -Dsolr.solr.home=/var/solr/data -Dsolr.install.dir=/opt/solr -Dlog4j.configuration=file:/var/solr/log4j.properties -Xss256k -XX:OnOutOfMemoryError=/opt/solr/bin/oom_solr.sh 8983 /var/solr/logs -jar start.jar --module=http

If not, it should be safe to remove the file:

sudo rm /var/solr/solr-8983.pid