Show me what your tables look like.

The first table is named tblCNBTrans.  It is like a check register.  I enter transactions from the bank.  These include date, vendor, category, deposit and charge.  This includes rent payments and expenses against the rental property.  The second table is for rent payments.  I append all records from tblCNBTrans into tblPaid that are category Rent Payment for a selected date range.  This table is named tblPaid.  This table has date, VendorId, Rent and Deposit.  If a renter makes two payments per month, then there are duplicate payments in the tblPaid for one renter.  In my current case, I have three renters with two payments.  My next table is tblNewMonth.  At the beginning of the month, I load this table with all renters and rent due for each renter.  Now my problem arises.  I update the rent paid by each renter from the tblPaid table into the tblNewMonth.  The problem is that as each record from the tblPaid table updates, it overwrites the value in the tblNewMonth.  So the first payment is updated and then the second record overwrites the first record.  So at the end of the update, I only have the rent for the last record, not the total of the two records for the three renters that have two payments for that month.  I need to be able to total all payments for each renter before I update tblNewMont.