SQL & PL/SQL

Dear all,
I have made a database procedure for inserting BLOB to database, using this code
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INSERT INTO CUST_SIGNATURE
(CUSTOMERID, SIG_NAME, MIME_TYPE, SIG_IMG)
VALUES
(p_custid, v_image_name, v_mime_type, empty_blob()) RETURN sig_img INTO b_lob;

f_lob := BFILENAME('SIG_SOURCE_DIR', p_filename);

dbms_lob.fileopen(f_lob, dbms_lob.file_readonly);

dbms_lob.loadfromfile(b_lob, f_lob, dbms_lob.getlength(f_lob));
dbms_lob.fileclose(f_lob);
COMMIT;
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Now i need to update the blob column with a new image.
The primary key for the table is combination of (CUSTOMERID, SIG_NAME).

I need to update the image for the selected (CUSTOMERID, SIG_NAME)

I searched in net. But couldnt get a proper answer.
Need something like this
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upate CUST_SIGNATURE
set SIG_IMG = to a blob
where customerid=p_custid, and sig_name= p_filename;
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Thanks in advance