In LSP.expert, we think it's important that a job and the linked invoice are consistant, so if you created a job in words, it's logic the corresponding invoice also mentions words.
If you wish to change the unit when invoicing, you will thus have to go and change the unit in the job itself, and then come back to the invoice and click on the three dots at the end of the job line and select 'Sync with job data'. This will update the invoice with the changes you made at the job level.
If you're using LSP.expert V1
All items on the invoice can be adapted manually. For that, you have to click on the pencil next to the line in question.
You'll then get this screen, where you can edit everything you want: