There are 2 prompts while booting, one asking to stop the sequence using ESC and a second one to stop it with Ctrl-C. Please wait till the second one shows and press Ctrl-C:
?[KAutoboot in 3 seconds... (press Ctrl-C to interrupt) <-----Press Ctrl-C
Main Menu 1. Boot [J]unos volume 2. Boot Junos volume in [S]afe mode 3. [R]eboot 4. [B]oot menu<-------------------------Select 4 5. [M]ore options
The next menu will display where you could force the switch to boot from a previous version:
Boot Menu 1. Boot [P]revious installed Junos packages <-----------------Select 1 2. Boot Junos in [S]ingle user mode 3. Boot from [R]ecovery snapshot 4. Boot to [O]AM shell 5. Snapshot [B]oot menu 6. [M]ain menu
The next menu will display where you could force the switch to boot from a previous snapshot:
Boot Menu 1. Boot [P]revious installed Junos packages 2. Boot Junos in [S]ingle user mode 3. Boot from [R]ecovery snapshot <---------------Select 3 4. Boot to [O]AM shell 5. Snapshot [B]oot menu 6. [M]ain menu
If the problem remains, you may use the USB format described below:
https://supportportal.juniper.net/s/article/EX4400-Format-Install-Procedure?language=en_US