Update with Join very slow

[Tatenda Chagonda]

I have 2 tables

Table A has 52 million records and table B has 30 Million Records

Table A has a primary key on ID number and Table B also have a primary key on ID Number

I want to update Table A with a column thats in table b and the query has been running for 2 days now. Please see the query below

UPDATE TableA a JOIN TableB b ON a.IDNo = b.IDNo
SET a.CellNo = b.CellNo
WHERE a.IDNo = b.IDNo