I wrote a Help Center article that details doing this using the Import / Export tool that you may want to look at. This is a good option if you don’t do this very often.
I’ve also created a simple screen that can perform this. This is a better option if you will be doing this often. I based the screen on the Entity table, but if you’re only going to be using this for customers, you may want to use the Customer table instead.
All you need is a grid that displays the table’s records, a drop down object that displays the SalesRep table’s records and a button. Make sure to add a checkbox column to the grid so that you can select the records you want to update.
The only actions you need on the button Start Loop Through Grid, Update Field In Table and End Loop Through Grid. Make sure to loop through Only checked rows or you will end up changing the value for all records. The update table in field action will the change the value of the SalesRep field to whatever you have selected from the drop down on your screen. Here’s what the actions look like:
I hope this helps and please let me know if you have any follow up questions.