I want to add new column in the sale invoice and also want to change the existing label like qty etc.
read the guide https://www.manager.io/guides/10411
this will require a custom theme. if you are familiar with liquid coding you can do it yourself, else you will have to hire someone locally to do it for you.
you can also search the forum for ideas as this have been discussed a few many times.
Also, if there is only one inventory or non-inventory item selected in a transaction, Manager will automatically change the heading Qty to the Unit Name you have saved for that particular item.