Sale deed query

Flat Owner - lalita
Flats- 2nd and 3 rd floor
In deed it was mentioned that 2rd floor owner have the car parking & 3rd floor owner does not have car parking.

I am purchasing the 3 rd floor but i also needs car parking, so as the owner of teh flats are same & they also agreed with this arrangement so please confirm what documents to be executed to include car parking also or what should be the lanugage of registry or what should be the process.
Thanks in advance.