You can use the client side API

if(this.value == 'value1'){
//alert('hahaha');
crm("oppo_description").caption("test");
}

what did you mean by "oppo_description"?

What should I put inside crm("")?

That is the name of the field that you wish to control.

See:  https://help.sagecrm.com/js-api/

And : https://www.youtube.com/watch?v=3HQQ8umkjr0

It does not work. I have attached my code below.

field name(current) = Type

field id(current) = emp_type

field name(need to be changed) = Company

field id(need to be changed) = com_id

Code :

if(this.value == 'val1'){
crm("Company").caption("New_Caption");
}

please give your suggestions.

It does not work. I have attached my code below.

field name(current) = Type

field id(current) = emp_type

field name(need to be changed) = Company

field id(need to be changed) = com_id

Code :

if(this.value == 'val1'){
crm("Company").caption("New_Caption");
}

please give your suggestions.

I made the assumption that you were working with a standard system screen e.g. Opportunity or Company.

Fields are referenced using their physical names including prefix  e.g. comp_name or oppo_description

Have you tried playing with the code in the console to check that you can in fact reference the client side API in your screen?

Following code worked! Thank You!

crm.fields('<field_name>').caption('<new_caption>');