Gnome-Subtitles

Subtitles can be edited using Gnome-Subtitles. 

https://launchpad.net/~pedrocastro/+archive/ubuntu/ppa

This PPA provides test versions of Gnome Subtitles. To install it:

1. Open Synaptic (System->Administration->Synaptic Package Manager)

2. Navigate to "Settings->Repositories->Other Software"

3. Click "Add"

4. Enter "ppa:pedrocastro/ppa" as "APT line"

5. Click "Add Source"

6. Click "Close"

7. Click "Reload"

8. Install or upgrade the "gnome-subtitles" package in Synaptic or using the command line: "sudo apt-get install gnome-subtitles"

Adding this PPA to your system

You can update your system with unsupported packages from this untrusted PPA by adding ppa:pedrocastro/ppa to your system's Software Sources. (Read about installing)

sudo add-apt-repository ppa:pedrocastro/ppa

sudo apt-get update

This version works great on Ubuntu 20.04.

SubSync

If editing is not needed because you only need to change the synchronisation. You can also try subsync. https://subsync.online/ 

 Download it frome here:

https://snapcraft.io/subsync

This program lests you synchronise subtitles using the titles from another srt file or it uses the subtitles of another language that is built inside the video file, or and that is great, it listens to the audio to synchronize the srt file. 

Alas it's only available as a snap. Drag and drop the srt file and the videofile and it tries automatically to sync the subtitles.

In Linux Mint you first need to install the snap store. look for that here.

After installation, subsync does not have access to removable media. To enable that open a terminal and enter:

sudo snap connect subsync:removable-media :removable-media