Option#1
Step 1) Go to Master > Account master > Account Master
Step 2) Select one Account and Press on Modify option (Alt + M).
Step 3) After that add the amount in the Opening Balance option and select the Cr. or Dr. option.
Step 4) after that Press Save (F2) option.

Option#2
Step 1) Go to Master > Account master > Account Master
Step 2) Press on Multiple Option.
Step 3) Open new form and then after Enter Opening Balance as per account select Cr. and Dr..
Step 4) Press Save (Alt + S) button.