Disk Partioning advice

This is the output from fdisk of my disk:

Disk /dev/sda: 465.76 GiB, 500107862016 bytes, 976773168 sectors
Disk model: Samsung SSD 860 
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

Disklabel type: dos
Disk identifier: 0xde067a1e

Device     Boot     Start       End   Sectors   Size Id Type
/dev/sda1            2048 717844479 717842432 342.3G 83 Linux
/dev/sda2  *    717844480 718893055   1048576   512M ef EFI (FAT-12/16/32)
/dev/sda3       718895102 976771071 257875970   123G  5 Extended
/dev/sda5       718895104 976771071 257875968   123G 83 Linux

The Linux OS that I’m currently using is on sda5

I want to install a new Linux OS on sda1 and be able to dual boot between the one on sda5 and the new one on sda1.

I’ve tried installing a new OS, but ‘bottled out’ when I wasn’t offered the option to install it on sda1!

I know just enough about disk partitioning to know that unless I’m very careful, I could lose the lot!

I’d welcome some advice, please.
Jim