Can anyone help on Creating a DNS host entry update script ?

What's the DNS solution you use?

If it's Microsoft or a BIND based solution you could use the native functionality. https://documentation.solarwinds.com/en/success_center/orionplatform/content/ha_set_up_ha_pools.htm 

But what you're using for DNS will certainly determine if you even need to script it. If you really do though, then you can take a look at the link below where it talks about the canned alert "Update High Availability Active Member DNS Record on AWS Route53 When Failover Occurs" - this can be used as a base to get you started.

https://documentation.solarwinds.com/en/success_center/orionplatform/content/ha-configure-alerts-for-other-dns-types.htm

Here's an example script I've previously used to update an Infoblox record for HA at the time when we couldn't use TSIG. No idea if it's close to what your needs are, but perhaps to get your mind jogging on the subject.

 SolarWinds High Availability update Infoblox DNS Record 

Ultimately though - you would need to create the script (if you DO require one... it's a last resort) as it's going to be unique to your environment.

Good luck

I am using other DNS. I have on prem DNS server in infra. Does solarwind require it update the active server IP address there during failover ?

You'd have to be a lot more specific than "other" dns, and would probably get your best results by asking for script help at a forum for whatever DNS solution you use.  

And yes, in multi subnet situations the failover requires that the DNS entry is updated to route to the server that is currently active.  Also the failover will not really be successful until that DNS update has propagated throughout your environment.  How long that takes depends on how your DNS is architected.