Check current RAC nodes 

<sxh bash>
root@grac41 Desktop]#  olsnodes -n -i -s -t
grac42    1    192.168.1.187    Active    Unpinned
grac43    2    192.168.1.253    Active    Unpinned
grac41    3    192.168.1.183    Active    Unpinned
</sxh>

Remove an instance 

<sxh bash>
[oracle@grac41 ~]$ srvctl  stop  instance -d grac4 -i  grac42
[oracle@grac41 ~]$ srvctl disable instance -d grac4 -i grac42
[oracle@grac41 ~]$ srvctl remove instance -d grac4 -i grac42
</sxh>

Remove instance from the database grac4? (y/[n]) y

Verify status 

<sxh bash>
[oracle@grac41 ~]$  srvctl config  database -d grac4
 Database unique name: grac4
Database name: grac4
Oracle home: /u01/app/oracle/product/11204/racdb
Oracle user: oracle
Spfile: +DATA/grac4/spfilegrac4.ora
Domain: 
Start options: open
Stop options: immediate
Database role: PRIMARY
Management policy: AUTOMATIC
Server pools: grac4
Database instances: grac41,grac43
Disk Groups: DATA
Mount point paths: 
Services: grac41,grac43,my_rac_svc
Type: RAC
Database is administrator managed
--> Instance grac42 successfully  removed
</sxh>

Add the instance:

<sxh bash>
[oracle@grac41 ~]$ srvctl add instance -d grac4 -i grac42 -n grac42
[oracle@grac41 ~]$ srvctl enable  instance -d grac4 -i grac42 
[oracle@grac41 ~]$ srvctl start instance -d grac4 -i grac42 
[oracle@grac41 ~]$ srvctl status  instance -d grac4 -i grac42 
Instance grac42 is running on node grac42

[oracle@grac41 ~]$  srvctl config  database -d grac4
Database unique name: grac4
..
Database instances: grac41,grac42,grac43
</sxh>