Update user meta without lose of old data

Is this is possible to update the user meta without lose the old data it have ?

For example :

i have the following data in my user meta field name user_documents

a:4:{i:0;s:36:"817fb3ec8e2412b141bcc40c1e16d4c9.jpg";i:1;i:91;i:2;s:27:"8769106-alone-wallpaper.jpg";i:3;i:92;}

this is about the attached files with the user_documents meta, if suppose a user upload more files i want to store it into the user_documents again but without lose the old meta which is above.