Tilda, not circumflex, for perispomeni in Ancient Greek

Could anybody help me to replace the standard circumflex with a tilda for perispomeni in Ancient Greek keyboard?
Thank you.

Could you tell me which keyboard you are referencing? (URL). We will need to check on the licensing if it’s one of the older keyboards in our repo.

You should never use the combining tilde (U+0303) for the Greek circumflex accent, only the dedicated COMBINING GREEK PERISPOMENI U+0342. So, please do not change the encoding via the keyboard (even though you could).
Instead, use a font which has the shape you want at U+0342 and in all the precombined Greek glyphs. An excellent such font is SBL Greek by John Hudson: freely downloadable from https://www.sbl-site.org/educational/BiblicalFonts_SBLGreek.aspx