It seems you are looking for one of the menu items underneath Patch program from the Edit item (on the main menu).
Personally before I start patching around I enable the "Number of opcode bytes" option and give it a generous value of 7 or so:
In your case you seem already to be set on a specific course of action and have made up your mind about which opcode to patch to what, so you can go right ahead and patch the
EB ?? to
C3 90, I reckon. The Change byte option could be exactly what you're looking for, but you need to know the hex representation of your opcodes to find the old one and replace it by the same number of bytes. Also, this seems to be limited to 16 bytes at a time, if I remember correctly.
Alternatively pick the Assemble... option and enter
ret. Any trailing bytes (your
jmp should be at least two bytes) will simply appear as non-code.
One point that may matter: this only applies to the IDB itself, not to the binary from which it got loaded. Use the last option in the popup menu for that.