If the option to Modify is available on the Main tab of the Assets section of Fixed assets, but not on the Financials, then the User Access Rights might be the reason:
Select the name of the user in question from the User list.
Click Company - Properties then select the Assets tab.
change the drop down options to 'Yes' for 'See Other Branch' and 'Modify Other Branch' and apply.
Close any outstanding fixed asset windows / tabs down, and now when you open them, you should be able to select Modify.