The first thing to say (already mentionened by @timemage) is that there is a large variety of "Digispark / Digistump" devices, both original, clones using the Micronucleus bootloader, even when considering only the ATTiny85 variants. Those variants, where the reset pin has not been re-configured as a GPIO pin, are easier to use. Otherwise, you firstly need a high voltage programmer to reset the ATTiny85 to its factory state, to restore the reset pin for configuring fuses etc.
The second thing is that the last post here: digistump.com/board/index.php/topic,2257.msg10556.html#msg10556 implies that 2.7v brown out, which you appear to need, is anyway already enabled (at least with the variant described by the author) That is, the fuse settings are: lfuse 0xE1 hfuse 0x5D efuse 0xFE.
Anyway, to change the fuses (or load the "Digispark / Digistump" (Micronucleus) bootloader) you need an ISP programmer (an Arduino or even another digispark can be configured as such). See: digistump.com/wiki/digispark/tutorials/programming. With the ISP programmer connected, you can then use AVRDUDE also to set the fuses.
In the worst case, you have to start with a factory condition ATtiny85 and go through the entire process of loading the "Digispark / Digistump" firmware onto it and configuring the appropriate fuse settings. See https://circuitdigest.com/microcontroller-projects/attiny85-ic-programming-through-usb-using-digispark-bootloader