Set list column field to read only

Hello,

I have list on my sharepoint online called Actual Budget which contains 3 column

Actual  Numeric Field With Default Value
Budget Numeric Field with Default Value
Target Numeric Field with Default Value

When user open the list to input the value I want Budget column uneditable the default value. No one should edit the defult value unless list owner goes to list setting and change the default value. How can I achieve this

Thanks

Kundan