How to write this magic item macro?

One of my players has a homebrew rapier that makes every 1 rolled on a d6 when sneak attacking to count as a 6, but i dont know how to make this macro. Help please

Sounds like you should make a rollable table for that die.  Have individual entry for the numbers 2-5 weighted at 1 each with 6 having a weight of 2.  Then, you would roll that for sneak attack damage.  If the table is called custom6, the roll would look something like [[1t[custom6]]].  Change the 1 to however many dice need to be rolled for the sneak attack damage.

I'm pretty sure OP would actually want to use:  [[[[1t[custom6]]]+[[1t[custom6]]]]] in the template.  If the sneak attack is more dice just add more to this, but be sure that you have the right amount of brackets.  

The rollable table will total, so [[2t[custom6]]] would calculate just like [[2d6]].  No need to separate them.

Another option would be to change the damage roll to: [[{1d6+1,{[[6]]}}kl1]] (I’m air-coding so the formatting may be off for the comparison - you may need double brackets around the 1d6+1 or something.) That way you don’t have to use a rollable table. I’ve found the rollable table list can become pretty bloated over time.