Conditional Select from Window Function
You need to put your current query into a subquery, and include an expression for the "lead last_reset" you want to put into the where clause.

Something like the following (untested):

select interval, bytes_diff
from (
  select 
    lead(update_time) over w - update_time as interval,
    lead(bytes_tx) over w - bytes_tx as bytes_diff,
    lead(last_reset) over w as lead_last_reset,
    last_reset
  from 
    table
  window w as (partition by t_id order by update_time)
)
where lead_last_reset = last_reset;


NB "lead"ing by 1 is so common that it's the default and can be left out.

NB2 All of your window functions are operating over the same window. So I made that explicit by defining the window in its own clause.