These directions have been vetted under the Cinnamon desktop environment (6.2.9) and libreCMC 6.2, though should be more or less applicable for a long time to come as little typically changes regarding these processes over long stretches of time.
1. Eject the microSD card from the router by pushing inward on the microSD card
2. Insert the microSD into a microSD reader or an microSD to SD card adapter and then insert into a SD card reader
3. Connect the card reader to the computer if using a card reader that is not built-in
4. In the lower left corner you'll find a menu button, click it, and a search box will come up, search for software and click on the Software Manager icon that appears, or on other distributions you may have an Activities icon in the top left corner you can do similarly with, but may have to open the Software Center, or similar app to install applications
5. In the Software Manager search for gparted, then click the gparted option when it appears, click the install button to install gparted
6. When gparted is installed click the launch app or find it in the menu and open it, note: you will need to enter your login password to open it, and you will need to be an administrative user on the system as well
7. In the top right corner of gparted there is a drop down showing the different disks on your system, make sure to select the one that is your microSD card, it'll probably say 3.69 GiB
and be the only drive of that capacity connected to the system
8. Right click on the partition labeled rootfs and select Unmount
9. Right click on the partition labeled rootfs and select the Resize/Move option
10. Drag the little play button to the right until you've got enough space for your need/application, do not drag it to the end as this may cause an issue resizing
11. Then click the Resize button in the lower right corner
12. Click the green check mark in the menu to apply the changes
13. Click the Apply button when warned about being sure you want to apply the the pending operations
14. Click the Close button
15. Remove the microSD card from the card reader and insert it into the microSD card slot on the router
16. Power up the router by plugging in the power, or unplugging the power and reconnecting the power
17. Give the router a minute or two to boot up
18. You should now be able to open a web browser and enter https://1921.68.10.1 into the address bar and hit enter
19. You'll probably get a warning message about a security certificate, you can safely ignore this, but you will need to add an exception to continue to the login page, to add an exception in firefox click the Advanced... button and then the Accept the risk and Continue button
Note: If your computer doesn't get an IP automatically from the router go to your network applet in the lower right corner (sometimes in the top right corner depending on the desktop environment/distribution). Click on the network applet and disconnect/reconnect. Also make sure you have disconnected from any wireless network you may be connected to, and an ethernet cable is connected between the LAN port on the router and the computers ethernet port.
20. You can also verify that it worked by logging in and scrolling down to the section labeled disks... here it'll show you've got gigabytes of space rather than 100s of megabytes of space assuming you've resized the partition into the gigabytes anyway